Gajim - Full featured and easy to use Jabber client
Dingtone - Dingtone is a free call and texting application for making unlimited calls and text messages to any number at the moment registered in any part of the world by any cellular company.
Empathy - Apps/Empathy - GNOME Wiki!
Textnow.com - It lets you text your friends, aka my only source of happiness
IM+ - Chat everywhere! IM+ is the most widely used mobile IM app! Connect all-in-one to MSN, Yahoo!, Goo...
FluffyChat - Open. Nonprofit. Cute ?