Ask YC: A good free solution for VOIP?

I am trying to find a good free software (preferably) or open source (at the very least) solution for VOIP in Linux.

I don't want to depend on anyone else - so it must a solution where I run my own servers (peer-to-peer would also be acceptable). Ideally, I would like a solution as good/practical and high-quality - once setup - as Skype.

XMPP and an integrated chat system would also be nice but are not required.

There seem to be literally dozens of options with no clear winner - which is why I am turning to your help, hoping that some of you had the same need previously and have some experience to share.