Aside from the obvious candidates such as Views, CCK etc, here are a list of 10 contributed modules that I currently use on each Drupal project.
So, in no particular order:
The admin module provides UI improvements to the standard Drupal admin interface. I've just upgraded to the new 6.x-2.0-beta4 version, and installed the newly-required Rubik/Tao themes. So far, so good!
- Better Permissions/Filter Permissions:
Basic permissions is a basic module which enhances the Drupal Permissions page to support collapsing and expanding permission rows. Filter permissions provides filters at the top of the Permissions page for easier management when your site has a large amount of roles and/or permissions.
- Better Formats:
Better formats is a module to add more flexibility to Drupal's core input format system.
- Clone module:
Allows users to make a copy of an existing item of site content (a node) and then edit that copy.
- Vertical Tabs:
Integrated into Drupal 7 core, this module adds vertical tabs to the node add and edit forms.
Context allows you to manage contextual conditions and reactions for different portions of your site. You can think of each context as representing a "section" of your site. For each context, you can choose the conditions that trigger this context to be active and choose different aspects of Drupal that should react to this active context.
- Node Picker:
A rewrite of the module TinyMCE Node Picker. Allows you to easily create links to internal nodes.
- Module Filter:
What this module aims to accomplish is the ability to quickly find the module you are looking for without having to rely on the browsers search feature which more times than not shows you the module name in the 'Required by' or 'Depends on' sections of the various modules or even some other location on the page like a menu item.
Quickly create Zen subthemes.
- Add Another:
Add another displays a message after a user creates a node, and/or displays an "Add another" tab on nodes allowing them to make another node of the same type. You can control what roles and node types see this feature.