Slack - A messaging app for teams who see through the Earth!
Shtuff Chat - Free Group / Team chat with persistent history.
Telegram - Telegram is a messaging app with a focus on speed and security. It’s superfast, simple and free.
Meet in Chat - Meet new people. Have fun sharing photos.
Mattermost - Mattermost is an open source alternative to Slack.
Kandan - Open Source private chat server. Your own private Chat server in minutes, plus additional features.