http://www.drupalcoder.com/story/466-must-have-and-popular-drupal-modules
Administration theme
Gives you more options to use the administration theme then provided in core.
Backup and Migrate
I use this to take a dump to store in SVN. This module allows you to exclude data from certain tables in the dump.
CCK
You should know what this does by now. Most used field types for me are Date, E-mail, Filefield and Imagefield
Clone
Asked by pretty much every client. Allows you to copy (clone) nodes. Client sets up a basis webform with fields required in every webform and clones that form to create new forms.
Flag
Flag or bookmark nodes, comments, users. Multi functional. Use it for bookmarks, flag as offensive, mark user as friend, ...
Fivestar
Allows you to rate nodes with "stars".
Google Analytics
Plug and play module to add Google Analytics tracking to your site. Allows you to track certain roles, ...
Internationalization
To much to explain. Basically allows you to translate nodes, menu items, taxonomy terms, ...
ImageCache
Scale, crop, desaturate, ... images. Create presets of image types.
IMCE
The best file manager for Drupal 6. Using IMCE Crop and IMCE Mkdir you have an easy way to manage directories and crop images.
Localization Client
Interface for translating pages. Shows a translation form on the bottom of your pages to easily translate sentences on that page.
Menu Block
A bit difficult to explain. But this is what's on the project's page: It provides configurable blocks of menu trees starting with any level of any menu. And more!
Mollom
Spam detection and content analysis. Created by Dries Buytaert, founder of Drupal.
NAT (Node Auto Term)
Automatically create a term for each node with the same title.
Nodequeue
Put your nodes in a queue and show the queue using views. Perfect for a "highlighted stories" block.
Pathauto
Automatically creates pretty urls for your nodes, users, ... based on title, date, fields, ...
Pathologic
Input filter which attempts to make sure that links to other content in your Drupal installation, as well as images and other media, will always work correctly
Restricted Search
Hide certain nodes from Drupal's search.
Scheduler
Schedule node (un)publishing.
Secure Site
Hide your entire site behind a login form. Perfect for staging and development sites to hide them from the public.
Spam Span
If you have to use mailto: in your textareas, this filter is a must have. It encodes your e-mail addresses to hide them from spammers.
Views
Again: you should know this one by now.
Webform
Create forms (contact forms, feedback forms, subscription forms, ...) without any programming.