Skype for Linux and OSX
Skype is by far the best voice-over-ip client I know. It even works well with NAT firewalls (how do they do that?!) Now there is also a Linux and OSX version available.
I found some packages for Debian. Just add these lines to your sources.list:
…and then do the usual “apt-get update; apt-get install skype”.
Unfortunately it does not work with proxies yet.
deb http://www.bootsplash.de/files/debian unstable main
deb-src http://www.bootsplash.de/files/debian unstable main