Modules to improve accessibility & usability; Or, Modules to keep users happy

Accessibility (& usability)

Out of the box, Drupal 6.x is relatively good with regard to accessibility, but there is room for improvement. Contributed modules are a bit more of a mixed bag. These modules are good (indeed, vital) for making various aspects of Drupal more accessible —and more accessible usually means more useable in general, too.

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.

User interface enhancements

Vertical Tabs (http://drupal.org/project/vertical_tabs)

Provides vertical tabs for supported forms such as the node edit page.

Taxonomy Super Select (http://drupal.org/project/taxonomy_super_select)

Requires Taxonomy
Changes the default taxonomy select box into checkbox or radio buttons.

Excerpt (http://drupal.org/project/excerpt)

In the node edit form, displays the teaser as a separate field from the Body field and makes it easier for the teaser to be something other than truncated body text.

User experience improvements

Better Formats (http://drupal.org/project/better_formats)

Enhances the core input format system by managing input format defaults and settings. This allows automatic selection of, for example, Full HTML for use by trusted content contributors, instead of such contributors having to manually select Full HTML for each general text field.

Wysiwyg (http://drupal.org/project/wysiwyg)

Requires installation of (non-Drupal) editor libraries.
Allows users to edit contents with rich text editors such as CKeditor and TinyMCE. Permits inclusion of more than one rich text editor in the same site.