Anologue - Disposable online chat.
Walkaround - Walkaround is a variant of Wave, based on the Apache Wave code base, that runs on App Engine.
EchoWaves - Opensource Social Group Chat written in Ruby On Rails
Rizzoma - Email and IM are linear contextless communication tools.
JabbR - A chat application built with ASP.NET and SignalR.
Slack - A messaging app for teams who see through the Earth!