Popular Modules in Drupal

1 Blogs
2 Forums
3 Events and event Calendar
4 Donations using Paypal
5 Online Catalogue
6 Online shopping cart without inventory Management
7 User Management
- Using Drupal Authentiation
- Using Windows Authentication
8 Job Search and listing

9 Contact Form
10 Site Search
11 Banner Advertisement
12 AdSense

13 Adsense Injector
14 Tiny MCE
15 FCK editor

16 Simple News for Newsletter signup
17 CiviCrm - for relatinal management system
18 File Upload Module
19 Archives
20 Amazon Tools - to display a amazon product on site

21 Add to Favorites
22 Mail to friend
23 Site Map
24 SEO Modules like Meta tags, HTML tidy
25 wiki style (small scale) with recent changes track
26 Flash into with drupal

Note: Each module has its own advantages and drawbacks.
Based on the client requirements, at times we need to do programming for custom module functionality.
Also a list of complete drupal modules is available at http://drupal.org/project/Modules