One of the great things about Drupal is its huge community of developers. Drupal.org hosts many, many modules to extend and enhance your website or application. Most people know about popular modules such as CCK, Views2, Panels, and XML Sitemap. But there are some really great modules that fly under the radar as well.
1. Secure [...]

Open Atrium is an open source web application for creating an intranet where co-workers/teams can collaborate easily.
It is built on the Drupal framework & has modules like:

blogs for each group
event calendar which can import feeds from iCal & others
documents, to store, collaborate & see revisions
Twitter-like shoutbox for sharing any info
case tracker which is a complete [...]

Magazeen (Drupal 5x & 6x) View Demo »

Absynthe (Drupal 5x & 6x) View Demo »