Anologue - Disposable online chat.
Walkaround - Walkaround is a variant of Wave, based on the Apache Wave code base, that runs on App Engine.
Shtuff Chat - Free Group / Team chat with persistent history.
Rizzoma - Email and IM are linear contextless communication tools.
Tlk2me - Tlk2me is simple real-time messaging platform which is designed exclusively for groups.
Slack - A messaging app for teams who see through the Earth!