Also WordPress MU › Blog Tool and Publishing Platform and bbPress Not sure if bbpress is official though. I think I got only the official links this time though
LOL I'm not sure - I know there is some kind of affiliation there though. MU is being ended and combined into WordPress itself soon.
BuddyPress is a suite of WordPress plugins and themes, each adding a distinct new feature. BuddyPress contains all the features you’d expect from WordPress but aims to let members socially interact. BuddyPress.org – WordPress Social Network Software
