Slack - A messaging app for teams who see through the Earth!
Telegram - Telegram is a messaging app with a focus on speed and security. It’s superfast, simple and free.
Flock - A faster way for your team to communicate
Cisco Jabber - Cisco Jabber lets you access presence, instant messaging (IM), voice, video, voice messaging, desktop sharing, and conferencing.
Mattermost - Mattermost is an open source alternative to Slack.
Flowdock - Chat & inbox for teams. One place to talk and stay up-to-date.