RetroShare [0] does this, too, and covers many other communication needs (mailing, chatting, forum, etc.) while being p2p/decentralized/public-key-encrypted/open source.
Also, it has terrible GUI and just too many features.
I think the word 'terrible' is an understatement.
It does not look like retroShare uses tor however, or at least I could not find mention of it with a cursory search.
Yes, it rolls it's own DHT to connect users and encrypts all communications between them end-to-end, but makes no attempt at anonymity for direct friend-to-friend connections once authenticated IIRC.