JIMP is a project mainly written in Java, it's free.
The Jitsik IM protocol. This is an eclipse workspace for the server.
JIMP is the Jitsik Instant Messaging Protocol. This protocol is defined by the specification listed below. The official Mac OS X client is still being developed, as well as the server. The specification is also still underway, and is not nearly complete.
You can read about the JIMP protocol here: JIMP protocol.