Drupal is one of the most flexible Content Management System available and Its been more than 18 months since i am using it and have tried and tested lot many  modules, in this post i try to categorize non core drupal 6 modules on the basis of my experience in must have and enhance your site categories. i am sure this list would change with time

Modules you cant live without

• CCK
• Views
• Administration Menu
• Pathauto
• Devel
• CAPTCHA
• WYSIWYG API
• SimpleNews
• Google Analytics
• Mollom

Enhance your site using the below

• ddblock
• Gallery
• Panels
• ImageCache
• Lightbox2