Sorry for not being able to post a link, but you can easily find the software with a search engine. It is cross-platform (open source, of course) and has builds for Win, Mac, FreeBSD and several Linux distros. The VoIP feature (plugin) is still somewhat under development, but I have tried it and it works (talking from Finland to China had a big lag, though). If you go and try it out immediately, it works by opening an instant messaging window with your friend and unchecking the red crosses over the two icons in the lower right corner of the window.

You can also share files with your friends. You connect to friends by exchanging PGP-certificates with each other.