Shtuff Chat - Free Group / Team chat with persistent history.
Trello - Infinitely flexible. Incredibly easy to use. Great mobile apps. It's free. Trello keeps track of everything, from the big picture to the minute details.
JabbR - A chat application built with ASP.NET and SignalR.
Notion - All-in-one workspace. One tool for your whole team. Write, plan, and get organized.
Smarsh Instant Messenger - smarshIM Mobile is a free application download that requires a paid account on the smarshIM service...
Slack - A messaging app for teams who see through the Earth!