Here is the list of top 50 drupal modules by average monthly downloads. This data is as of 2009. With Drupal’s popularity increasing the download data may vary. But the top downloaded modules are not likely to change much.

Drupal Module / Theme
Average monthly Downloads

1. 
   Views
   300,000
2. 
   Content Construction Kit (CCK)
   280,000
3. 
   Token
   250,000
4. 
   Pathauto
   210,000
5. 
   Administration menu
   140,000
6. 
   Image
   140,000
7. 
   ImageField
   130,000
8. 
   FCKeditor – WYSIWYG HTML editor
   130,000
9. 
   ImageAPI
   120,000
10. 
    Date
    120,000
11. 
    IMCE
    120,000
12. 
    ImageCache
    120,000
13. 
    FileField
    115,000
14. 
    Poormanscron
    115,000
15. 
    Google Analytics
    115,000
16. 
    CAPTCHA
    100,000
17. 
    Advanced help
    100,000
18. 
    Webform
    80,000
19. 
    Update Status
    80,000
20. 
    Meta tags
    80,000
21. 
    Link
    78,000
22. 
    Lightbox2
    77,000
23. 
    XML Sitemap
    64,000
24. 
    jQuery Update
    60,000
25. 
    TinyMCE WYSIWYG Editor
    60,000
26. 
    Devel
    58,000
27. 
    Content Templates (Contemplate)
    58,000
28. 
    Voting API
    56,000
29. 
    Panels
    56,000
30. 
    Calendar
    52,000
31. 
    Tagadelic
    52,000
32. 
    Backup and Migrate
    52,000
33. 
    Printer, e-mail and PDF versions
    52,000
34. 
    Global Redirect
    51,000
35. 
    Image Assist
    51,000
36. 
    Fivestar
    50,000
37. 
    Wysiwyg API
    50,000
38. 
    Zen
    40,000
39. 
    DHTML Menu
    40,000
40. 
    Email Field
    40,000
41. 
    Event
    40,000
42. 
    Javascript Tools
    40,000
43. 
    Internationalization
    40,000
44. 
    Nice Menus
    36,000
45. 
    Simplenews
    36,000
46. 
    Thickbox
    32,000
47. 
    LoginToboggan
    32,000
48. 
    Site map
    30,000
49. 
    External Links
    28,000
50. 
    Page Title
    27,000