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I know that the new Tor’s Bleep online chat service does, but that is in invitation only alpha at the moment. Here is the link where you can submit your email address to be put on the invitation waiting list. http://labs.bittorrent.com/experiments/bleep/
There is also invisible.im which is specifically designed to allow people like whistleblowers to share information with being compromised. You might want to keep an eye on it...it’s still in development. http://invisible.im/#
Another option is the i2pMessenger that uses i2p anonymization. https://geti2p.net/en/docs/how/tech-intro