It seems like every company has their own implementation of these - are there any open source versions? I'm strongly contemplating throwing together a gem for rails this weekend.

EDIT:

I found http://github.com/bvandenbos/redis_feature_control which handles features globally (either on or off for everyone). Unfortunately doesn't handle things on a per user/session/etc basis though - although it shouldn't be too hard to add that feature using Redis sets.