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.
Telegram - Telegram is a messaging app with a focus on speed and security. It’s superfast, simple and free.
Facebook Messenger - Facebook Messenger is a faster way to message.
WhatsApp - WhatsApp Messenger: More than 1 billion people in over 180 countries use WhatsApp to stay in touch with friends and family, anytime and anywhere.
Digsby - Tagged makes it easy to meet and socialize with new people through games, shared interests, friend suggestions, browsing profiles, and much more.
Signal - Fast, simple & secure messaging. Privacy that fits in your pocket.