1. Linphone – Linphone is an internet phone or VoIP much like Skype. t is released under the GNU GPL v2 license and commercial support is available as well. It comes in binaries for Debian-based distros or build from source.
2. Ekiga – Ekiga was formerly known as GnomeMeeting, which many have heard of. GnomeMeeting used to come with lots of distributions and Ekiga is still seen in several. It provides “SoftPhone, Video Conferencing and Instant Messenger application[s] over the Internet” and supports SIP and H.323. Ekiga is released under the GPL and comes in binaries for lots of distros and in source code.
3. Empathy – “Empathy is a messaging program which supports text, voice, and video chat and file transfers over many different protocols.” This one is probably most well known because of its inclusion in Ubuntu. 3.1 was released May 9. It is released under the GPL and comes in binaries for Ubuntu.