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.
aTalk - An encrypted instant messaging with video call and GPS features for android.
Telegram - Telegram is a messaging app with a focus on speed and security. It’s superfast, simple and free.
Talk360 - Talk360 is a best-in-class communication utility that offers a convenient way to connect with loved ones globally.
Adium - Adium is a free instant messaging application for Mac OS X that can connect to AIM, MSN, Jabber, Yahoo, and more.
FluffyChat - Open. Nonprofit. Cute ?