First, getting started with content in Drupal: Node Types.
Also see the docs on Drupal's Taxonomy system.
Also see the great IBM intro to developing a collaborative web site.
Most of the non-essential core modules listed here will be useful (comment, forum, node, profile, search, statistics, taxonomy, tracker, user).
Here's a list of interesting contributed modules I'd like to learn more about:
- Simplenews: Simplenews is a simple newsletter module which allows both anonymous as well as registered users to subscribe to different newsletters.
- Massmailer: manage mailing lists (based on PHPList
- Listhandler: synchronize mailing lists and forums
- Organic Groups: Enable users to create collaborative groups (incompat. w/ taxonomy?)
- Organic Groups List Manager: integrated mailing list/forum for OGs.
- Node Vote: a node voting system
- Interwiki: wiki syntax for linking
- URL Filter: automatically turn URL text into hyperlinks
- User Points: users gain points as they do certain actions
- Tagadelic: weighted tags in a cloud
- Privatemsg: an internal messaging system
- Pathauto: automatic path aliases for nodes and categories
- Biblio: manage lists of scholarly publications (including .pdf upload)
- SPAM: tools to stop unwelcome posts
- Services: standardized API for drupal
- Google Analytics: free advanced web stats