The token module and tokens in core and contributed module provide an essential feature to Drupal internals, that enable site developers to use variables for site information site as [node:title] to reference the name of content when performing site building.

The toolbar themes module qualifies as an essential administration GUI functionality for all Drupal 8 websites. When used along with the admin toolbar module, together they provide an optimal admin GUI menu configuration. With the two foregoing modules, the admin GUI menu uses less windows area as well. 

This module was developed by New Zealander Jeff Burnz, the one of the various gurus of Drupal theming.

The Trash Module implements a Windows/MacOS style trash can model for content deletion, so that deleted content can be restored if desired, or later permanently deleted. Most Drupal websites probably don't implement this module because it is possible to unpublish content so that it is not visible to anonymous visitors, or with one of various access control modules it is possible to restrict access to content so that it is only visible to specific user roles. The Trash Module depends on the Multiversion Module. When used together with modules such as the Workspace Module, the Deploy Module, and Conflict Module (handles content revision conflicts), this suite of modules makes it possible to construct reliable multi-user site content management facilities within a Drupal website deployment. 

The Twitter Block Module enables site builder to present one of more Twitter tweet timelines in blocks on a Drupal website. this is more of various similar modules that provide similar functionality.

The Ultimate Cron Module implements enhanced cron background job handling from within Drupal website deployments. This module implements core must have functionality for every Drupal website. 

The URL Redirect module implements an admin GUI for redirect URLs from one to another, and can specify behaviors based upon roles and even specific users. This module implements redirection of primarily internal URLs based on Drupal roles. This is probably handy for situations where some authenticated users should have access to certain URLs, and other should not, but should be redirected to some other content when they attempt access the restricted content. 

The User IP Log Module is a simple module that logs the IP address of users, recorded during the login event, to a table in the Drupal database.

This module, the username validation module, implements restriction of the contents of Drupal user names. I have been using this module to prohibit spaces and special characters in Drupal user names, restricting user names to alphanumeric characters only. This Drupal 8 version of this module works better than the Drupal 7 version. There is a module for Drupal 7 called custom username validation that works properly and better on Drupal 7, but it has not been ported to Drupal 8. 

Currently a patched version of this module is required, which implements prohibition of spaces in Drupal user names. Notably this should be distinguished people name, for which the Real Name module provides and interface for implement real word people names, in addition to login user names, in Drupal websites.

The Video Embed Field Module implements Drupal contents fields that make it really easy to embed and display video from sources like youtube and vimeo. This module is deployed on this very website to implement video carousel blocks that contain carousels of videos on Drupal website development and Drupal website building. A Drupal 7 version of this module is also available.

The View Modes Display Module implements preview of entity rendering for use during content creation. View Modes Display is a helper module to preview view modes for your entities. When working on a site with many content types and view modes, it becomes tedious to find out where to view the rendered entity in a given view mode. This module aims to solve this by providing an additional tab "Preview Display" on the entity itself. On the "Preview Display" page, all view modes with custom settings will be rendered in the enabled theme. A contextual link is also added for nodes to make it easy to link to the "Preview Display" from anywhere the entity is shown.

This module, the Views Accordion Module, transforms a Views list into an accordion format, wherein only the list item with the focus is displayed in full, while all the others display only a title or summary. 

This module implement a blogspot style archive list block with yearly, monthly categorization of nodes. 

The Views Breadcrumb Module helps to correctly build breadcrumbs fields for Views. What is really needed for Drupal 8 is a port of the Drupal 7 Crumbs Module for which it doesn't appear any development work has been started. 

The Views Contextual Range Filter Module is a Views plugin that implement contextual filtering by range rather than a single value. 

The Views Cumulative Field Module implements a Drupal views field handler that enables the field to calculate the cumulative value of another field being display by the view. Among the use cases for this a views that are using a charting plugin to display data points in addition to the accumulated value. This is an occasional use sort of module that is useful to know about at the moment one is building a Drupal View that needs the functionality this module implements. This is the sort of module of which there could/should be thousands to implement detail level features for Views.

The Views Data Export Module implements data export from Views. This is a useful module for websites that present tables of information, reports for specific application oriented data on a website, that want to be able to export the contents of the views output. 

The Views Extras Module implements additional arguments for views which are then available as contextual filters, such as session information, cookie information, and Drupal tokens.

This module is the - one to many relation - Views functionality that is seems like Drupal beginners in the Drupal forums have asking about for years. The module has been available for Drupal 7 for years, and has a beta release available for Drupal 8. With this module it is possible to easily implement an order, order items style of views table with a table within a table providing the list of records associated with the many table from the one table in the one to many relationship. As the module description states: the module enabled a site builder to embed a view within another view and pass arguments from the master view to the subsidiary view. 

The Views Flipped Table module implements the ability to generate a table where the rows a displayed as columns and the columns appear as rows. This is the sort of table style one sees with Views such as product comparison tables. 

This module is a View plugin module that provides a tabular output with the fields vertically rather than horizontally. This style of table is used most often for features like production comparison tables. 

From the module description:

Use Views Global Filter on sites that feature multiple Views. You want the views to be driven by the same filter selection, for instance a country, a date range, a search term or even proximity to a post code. These views may be on the same page or scattered across several pages on your site, however you want to offer the visitor the convenience of setting the filter for all these views only once, for instance when they first land on your site.

Views Global Filter implements the picked once, operates site-wide (POOS) principle. This means you can place anywhere on your site a selection widget (drop-down, check boxes etc) to globally filter all or some of your views. When the user picks a value from the global selector, the value is invisibly passed as a contextual filter to whatever views you like, even when these reside across multiple pages. This avoids you having to duplicate filters and avoids the visitor having to re-select their choice as they go from page to page.

This module is probably useful for sites that want to implement various global preferences that apply throughout a site. However, its effectiveness only applies to Views. 

The Views Jump Menu Module implements a Views display style that creates an HTML select element using the output of the View. 

The Views List Sort Module is a Views plugin that enables a Drupal View to be sorted by the values of a list field. This is useful for sorting a Views in an arbitrary order when the values of a list are not in alphabetical order.

The Views Merge Rows Module fixes duplicate rows that sometimes occur when a View is built using various types of table joins.

The Views Natural Sort module implements views sorting filter plugin that provides natural sorting for view display entries that may include prefix terms like The and A, which are not fundamental to the alphabetic identity of the field being sorted within the View. 

The Views Random Seed Module provide a random number seed for Drupal Views, which can be used by Drupal site builders for a variety of purposes.

(from the module's description page)

A Views Reference Field, in conjunction with the Paragraphs module, makes for a very powerful content management system, allowing addition of just about any kind of content into an entity page.

In Drupal 8, Views are now identified as entities and the core Entity Reference Module is able to reference Views, however not Views displays. This module leverages core entity reference module functionality to add the display ID so that a View can be rendered in a field formatter.

This module does the same as View Reference, for which there is currently no Drupal 8 release or any messages on the project page regarding Drupal 8. Because Views Reference leverages Drupal 8 core functionality, this module is quite different from the other module and there is no upgrade path.

Explanation and features:

A Viewsreference is a entity reference field that you add to an entity via the 'Manage fields' tab of an entity.
With Viewsreference you can enter content unique per item of content (eg node) so that one item might have one view and another item might have a dozen views.
If your site has a lot of views, the viewsreference field provides a brilliant substitute to content placement widgets such as context and drupal block admin.
With viewsreference fields you can delegate permissions easily to an editor to add views to a page in a much more intuitive way than either option in #3.
Views reference field currently allows you to set arguments and a title field. The field now has an extra column called data which can be used in an extension of this field type to apply more settings as discussed. The 2.x branch adds changes to already leverage the data field for the available title/argument fields.

This module is a Views plugin that implements tables with a secondary row to each primary table row. This feature is used in the listing of modules that includes list module information.

The Views Send Module works in conjunction with the mail system module to implement GUI mass mailing system list. There are setup instructions here.

The Views Simple Math Field Module enables a site builder to perform basic arithmetic on two fields within a Drupal view. This module fills and omission that used to exist in Drupal 7 Views. The View must include two fields with numeric content. Then the Global: Simple Math Field can be added to the view and configured to perform arithmetic on the two numeric fields.

This module provides a slide show generator for the Views subsystem. This isn't the only way to create slide shows, but it is inherently helpful for Drupal site builders who don't have access to other slideshow implementation components. For site builders with access to custom slideshow block implements, this module becomes obsolete. On this very website, Views Slideshow, wouldn't be used because this site uses much more powerful image and video sliders that deploy more feature filled javascript library enhancements. 

With this module it is possible to split the display styling (View Mode) of a view between two different view styles. This module only works for content display format, not with fields output or other display format. Although this module implements a convenience for building Views, its application does not seem likely to have frequent application.

The Views System module extends the Views module and allows to create customized lists (pages, blocks) of modules, themes, and theme engines that are or have been installed in Drupal's file system. This module also has a helpful Drupal 7 version.

The Views Token Argument Module enables site builders to use a Drupal token as a Views contextual filter. The sorts of tokens available relate to the data in the View, such as field ID, node ID, and so on which can all be used as a contextual filters. This module is similar to the Views Extras Module. Like many modules such as Views plugin modules for Drupal 8, this module has a limited deployment count, which appears to be an indicator of the limited nature of Drupal 8 site deployments generally.

The Views Tools module implements export import services for Views.

The Visitors Module records a variety of information about both anonymous and authenticated Drupal site visitors, and makes the data available via admin GUI reports. It seems odd that this module hasn't had a release since 2014, which begs the question of the extent of testing it has received for compatibility with the newest releases of Drupal core, approach 8.4.x.

The Webform Module seems like the king of forms implementation for Drupal. It is well supported by its developer, including numerous youtube tutorial videos about how to use and deploy its many features. 

The Webform Views Module enables site builders to create Drupal Views that display Webform contents and their results. 

The Wordfilter Module is a text filter enhancement module that can be used to implement features like profanity prohibition. It can also be used to perform other types of word substitutions (replacements), for example, to automatically catch and correct certain spelling errors throughout a website's text automatically. 

The Workspace Module is part of a suite of modules that implement Drupal project and content deployment workflow. This module depends on the Multiversion Module, which enhances all entities so that they are revisionable.

This module, the xmlsitemap module, generates a sitemap in xml format that conforms to a standardized specification for submission to search engines.