Wave in a Box is an implementation of most of the same technology that went into wave.google.com, including all of the operational transform real-time sweetness, a web client, a command line client (that's right, a vim-inspired version of wave) and support for federation (ie. interoperability of multiple servers on different domains).

You can get all of this, open source, right now. Just head over to:

https://sites.google.com/a/waveprotocol.org/wave-protocol/code/installation

and get going. Building and running of all the components is very straightforward and can be done in about 5 commands. A copy of Jetty is included in the build.

The only missing piece is a persistance layer, which according the FAQ is an active priority for the team.