Empathy - Apps/Empathy - GNOME Wiki!
TextNow - TextNow is a mobile virtual network operator (MVNO) that provides a free phone service to users by using other cellular networks coverage.
Gajim - Full featured and easy to use Jabber client
Google Voice - Google Voice gives you one number for all your phones, voicemail organized in an email format, free...
IM+ - Chat everywhere! IM+ is the most widely used mobile IM app! Connect all-in-one to MSN, Yahoo!, Goo...
Burner - Disposable phone numbers -- now with MMS photo support