TextNow - TextNow is a mobile virtual network operator (MVNO) that provides a free phone service to users by using other cellular networks coverage.
Empathy - Apps/Empathy - GNOME Wiki!
Google Voice - Google Voice gives you one number for all your phones, voicemail organized in an email format, free...
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.
Burner - Disposable phone numbers -- now with MMS photo support
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.