Ricochet is an open source peer-to-peer instant messaging system built on Tor hidden services. Your login is your hidden service address, and contacts connect to you (not an intermediate server) through Tor. The rendezvous system makes it extremely hard for anyone to learn your identity from your address.
An audit by NCC Group through the Open Technology Fund validates Ricochet's security and provides a great outline of areas to improve in the near future (https://ricochet.im/files/ricochet-ncc-audit-2016-01.pdf).
Ricochet is not affiliated with or endorsed by The Tor Project.