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
IM+ - Chat everywhere! IM+ is the most widely used mobile IM app! Connect all-in-one to MSN, Yahoo!, Goo...
Sipdroid - Free and open-source SIP/VoIP Client
Pinger - Pinger lets you text and talk free with the people you care about most, anywhere on the planet.