BuddyPress.org

By the way, as an aside, I’ve been an active member of Metafilter, which I assume most of you have heard of (perhaps erroneously), closely watched it evolve for about 10 years now, and think of it as one of the best, smartest, most well-run semi-open communities on the web, and I believe a lot of lessons about how to keep things as simple and clear as possible (some say too simple, but I tend to disagree) while offering as much functionality as possible can be learned from the decisions Matt Haughey has made there.

It may be old-school, but looking only or even primarily to Facebook and Twitter and the like for inspiration can be a mistake, I think.