[khed]

Bitmessage hides the metadata by sending a given message from one person to another person to everyone in the network (it obviously has scaling problems!). Because everyone gets every message you can't really see the metadata.

I2p-bote routes the messages through multiple intermediaries with layered encryption so that no intermediary knows both the ultimate sender or ultimate recipient.

Pond works by running servers over tor (a mixnet). When you send a message it doesn't go to the server you have service through it goes directly to the recipients server.

Ricochet sets up your own server on tor that you are hosting.

There is also Pir-Tor. It attempts anonymity by using the secure multiparty communication combined with PIR to attain provable anonymity.