Anologue - Disposable online chat.
Unsee - The ability to open a chat session and message then delete it from history forever just like tlk.
EchoWaves - Opensource Social Group Chat written in Ruby On Rails
Chatango - Add free live chat to any web page. Chatango is a live group on your site
JabbR - A chat application built with ASP.NET and SignalR.
Walkaround - Walkaround is a variant of Wave, based on the Apache Wave code base, that runs on App Engine.