I've recently found that it makes a pretty kick-ass campaign website, as well.
Essentially, this is a free (if you have web space), open-source alternative to Obsidian Portal. It is built on the Drupal CMS, so it is extremely extensible. Built in features include:
- blogging - Anyone on the team can write a blog post. So far, my group has used this to discuss character ideas, generate setting questions, and discuss rules.
- microblogging - This is kind of twitter like. My group hasn't used it for anything useful, but it is mildly amusing. I might dedicate it to off-screen, in-character discussions down the road. or something.
- a calendar/event system - good for scheduling games. If you are running a modern game, I suppose you could use it to track in-game events as well. That could be interesting.
- Notebook: a wiki alternative - This requires some explanation. It is based on Drupal's Book module, and it allows hierarchical structuring of pages. I've set up one notebook for rules and one for setting. I'll probably also create one for NPCs. You can use this for just about anything you can use a wiki for... though it isn't precisely the same.
- case tracking - I haven't enabled this. It might be useful as a quest log, but is probably overkill.
All content is, by default, restricted to your group, and you can allow group members to create all and/or edit the different types of content (or not, if you prefer). Commenting is enabled by default on the blog, but you can add it to the Notebook as well. You can also use Drupal's powerful taxonomy system to categorize content across blogs and notebooks.
There are some additional features you can download for Open Atrium that look pretty cool, such as an ideation tool and integration with Graphmind. There are also thousands of other modules for Drupal, but they will require varying levels of configuration and customization to work the way you want them to work.
So far, I'm a pretty big fan of the way this is working out. I have no complaints about Obsidian Portal, but this gives me a bit more control over my own content.