This is a very nice list
http://en.wikipedia.org/wiki/Comparison ... P_software
Unfortunately it does not say which programs do support direct connections from Ip to Ip without server (for example WengoPhone can`t). Which programs can?
Some points I prefer:
- very good sound quality
- Open Source prefered most
- freeware also prefered, commercial last resort
- stun support is nice but not important because we have already overcome firewall problems
- built in secure end-to-end encryption and authentication with preshared key or certificates & hash or public/private keys (secure against man-in-the-middle-attacks) also very nice