BuddyPress.org

Because BuddyPress is not even 1.0 yet, it means that a lot of the code has evolved and changed quite a bit since the beginning. There have been almost 1400 code revisions since tracking started.

Choosing to develop a plugin for BuddyPress right now means staying pretty close to the bleeding edge of the development cycle because chances are that your plugin could be broken tomorrow through no fault of your own. Most current plugin authors are aware of this, and there has been pretty good response time to bugs and fixes. Usually within a day or so.