Essentials

These contributed modules should be installed in every site.

Content Construction Kit, aka CCK, aka Content (http://drupal.org/project/cck)

Allows administrators to add custom fields to content types. Included field types are text, number, node reference, and user reference.

Views (http://drupal.org/project/views)

Creates customized lists and displays from already entered content. Along with CCK, this is the heart of adding content once but being free to display it multiple places and multiple ways. (For those familiar with relational databases, CCK & Views let you use the power of relational databases to wrangle your content, while still keeping content adding and editing as simple as word processing and online shopping.)

Semantic Views (http://drupal.org/project/semanticviews)

Requires Views.
Permits UI management of HTML markup used to display views, which in turn allows use of semantically meaningful markup. For example, it lets you use <h2> instead of <div> for the titles of content pages listed in a view, and just in general cleans up the <div>-soup (and class attribute overload) normally created by most Views output styles. Semantic markup is absolutely necessary for accessibility. People using screen-readers cannot see that this <div> has larger text than that <div> —indeed, their screen-readers usually don't even tell them that the content is in different <div>s. For a visual demonstration of the problem, see This is your family on "Style: Unformatted" and This is your family on "Style: Semantic Views". As an added benefit, the HTML markup is much more concise and intelligible with Semantic Views, too.

Poormanscron (http://drupal.org/project/poormanscron)

Makes sure cron.php is run regularly, which is necessary to take care of various background maintenance tasks.

Date (http://drupal.org/project/date)

Requires CCK. Recommends jQuery UI for popup calendars and time entry widgets.
Defines CCK date/time fields and widgets. What makes this module essential is that it also provides a Date API submodule that provides superior date & time handling than in Drupal core.

Module Grants (http://drupal.org/project/module_grants)

Enables access control for unpublished content and allows modules that operate on access grants to work together in the expected way.

These optional core modules should be enabled in every site.

Update status (update)

Checks the status of available updates for Drupal and your installed modules and themes. Enabled by default (unless deselected during installation).

Menu (menu)

Allows administrators to customize site navigation menus. Enabled by default.

Path (path)

Allows users to rename URLs. Human-friendly URLs (e.g.,"about" rather than "node/72") are important for accessibility, usability, and SEO.