Signal - Fast, simple & secure messaging. Privacy that fits in your pocket.
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.
Conversations.im - XMPP client designed with ease of use and security in mind
Telegram - Telegram is a messaging app with a focus on speed and security. It’s superfast, simple and free.
Telegram X - Meet Telegram X for iOS and Android – two new official apps that offer more speed, longer batter life and smooth animations.
Adium - Adium is a free instant messaging application for Mac OS X that can connect to AIM, MSN, Jabber, Yahoo, and more.