sipc

http://www1.cs.columbia.edu/~xiaotaow/sipc/

SIP softphone from Columbia University

Runs on a range of platforms: Windows 95/98/NT/2000/XP, Linux and Solaris.

sipc does not provide audio and video functionality itself; rather, it uses external media application for handling media streams. Currently, it uses rat (Robust Audio Tool) as its audio application for both Unix and Windows version, vic as the video application, wb (for Unix) and wbd (for Windows) as white board application.

Free for education and research users only.
Commercial version is available from: CounterPath