As Jeff says - Gizmo is a SIP-based VOIP client. Like Skype, but based on open standards. It works great on Ubuntu - just grab the Debian download and do
sudo dpkg --install /home/pat/download/gizmo-project_184.108.40.206_i386.deb. I'm still tyre-kicking, but what I've see so far looks good. Plus, it even runs on my Nokia 770, turning that little beastie into a wifi phone - result!