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
Views
300,000
Content Construction Kit (CCK)
280,000
Token
250,000
Pathauto
210,000
Administration menu
140,000
Image
140,000
ImageField
130,000
FCKeditor – WYSIWYG HTML editor
130,000
ImageAPI
120,000
Date
120,000
IMCE
120,000
ImageCache
120,000
FileField
115,000
Poormanscron
115,000
Google Analytics
115,000
CAPTCHA
100,000
Advanced help
100,000
Webform
80,000
Update Status
80,000
Meta tags
80,000
Link
78,000
Lightbox2
77,000
XML Sitemap
64,000
jQuery Update
60,000
TinyMCE WYSIWYG Editor
60,000
Devel
58,000
Content Templates (Contemplate)
58,000
Voting API
56,000
Panels
56,000
Calendar
52,000
Tagadelic
52,000
Backup and Migrate
52,000
Printer, e-mail and PDF versions
52,000
Global Redirect
51,000
Image Assist
51,000
Fivestar
50,000
Wysiwyg API
50,000
Zen
40,000
DHTML Menu
40,000
Email Field
40,000
Event
40,000
Javascript Tools
40,000
Internationalization
40,000
Nice Menus
36,000
Simplenews
36,000
Thickbox
32,000
LoginToboggan
32,000
Site map
30,000
External Links
28,000
Page Title
27,000