Shtuff Chat - Free Group / Team chat with persistent history.
Slack - A messaging app for teams who see through the Earth!
JabbR - A chat application built with ASP.NET and SignalR.
Mattermost - Mattermost is an open source alternative to Slack.
Smarsh Instant Messenger - smarshIM Mobile is a free application download that requires a paid account on the smarshIM service...
Telegram - Telegram is a messaging app with a focus on speed and security. It’s superfast, simple and free.