Linphone - Linphone is an internet phone or Voice Over IP phone (VoIP).
Empathy - Apps/Empathy - GNOME Wiki!
Zoiper - Cross-platform VoIP softphone dialer for SIP and IAX systems
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.
Sipdroid - Free and open-source SIP/VoIP Client
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.