Telegram - Telegram is a messaging app with a focus on speed and security. It’s superfast, simple and free.
Trillian - Trillian is a decentralized and federated instant messaging platform that lets your whole company send private and group messages, keep tabs on what co-workers are doing, share files, and much more.
Signal - Fast, simple & secure messaging. Privacy that fits in your pocket.
Skype - Stay in touch with your family and friends for free on Skype. Download Skype today to chat and call on desktop and mobile.
Pidgin - Pidgin is an easy to use and free chat client used by millions. Connect to AIM, MSN, Yahoo, and more chat networks all at once.
Slack - A messaging app for teams who see through the Earth!