Linphone

http://www.linphone.org/ (French)

Linphone is an open source (or free software) SIP Softphone, with support for several codecs.
It runs using a gtk interface linux and windows.
Besides the graphical interface, some command line interfaces are available, called linphonec and linphonecsh, the lastest being easier to use from scripts.

Features

• Voice, video, and text instant messaging
• SIP protocol – interoperate with most SIP-compatible phones and proxies
• SIP proxy support: registrar, proxies, with digest authentication
• IPv6 support
• Video codecs: H263-1998, MP4V-ES, H264, theora
• Audio codecs: wideband speex, gsm, speex, g711, ilbc
• Video format up to SVGA
• Echo cancelation

Portability

• Linux on x86, x86-64, arm, and blackfin processors
• Windows XP and vista

It uses the GNU oSIP stack.

If you want to use Linphone, please make sure that you are using ALSA sound drivers and not OSS.
Linphone should work with OSS, but most of the time you will get the most mysterious erors you have ever seen, and _if_ you hear the sound, it will be slow and bad quality with OSS drivers. With ALSA sound drivers, sound quality and response are perfect.

VOIP Phones