TorChat is a peer to peer instant messenger with a completely decentralized design, built on top of Tor's location hidden services, giving you extremely strong anonymity while being very easy to use without the need to install or configure anything.
Please see the wiki page: https://github.com/prof7bit/TorChat/wiki for more information.
Downloads can be found in the section "Download Packages" on the following page: https://github.com/prof7bit/TorChat/downloads
The current stable version is 0.9.9.551 (released 2012-02-06)
Change log: https://raw.github.com/prof7bit/TorChat/master/torchat/src/changelog.txt
(End users: please ignore the two buttons "download zip" and "download tar.gz" on the top of the download page, these are only source code snapshots for developers and as an end user you most likely simply want to download one of the packages.)