In watching Jay Phillips do his great presentation here today at AstriCon about Ruby and his Adhearsion package, I found myself wondering what the interesting little softphone was that he was using. It turned out to be "Zoiper", an IAX or SIP softphone that was previously called "Idefisk". (I can understand perhaps why they changed the name... "Idefisk" does not exactly roll off your tongue.) There turn out to be two versions (comparison chart here): a free version and a "Zoiper Biz" version which includes more functionality and starts around 30 euros.
Clearly built for Asterisk, it was interesting to note that it supports both SIP and also Asterisk's own IAX protocol. Anyway, I just thought I'd share that this softphone is out there if you were not aware of it.