I've used MSN Messenger for talking with some of my relatives in Europe and seemed to work pretty well. Just make sure you don't have any sort of firewalls at home (routers) or you'll have to configure certain ports to be opened.
I've also used webcam with Messenger and on a 320x240 resolution, using cable on one side and dsl on the other, it seemed pretty smooth.

Now I know you can have unlimited talk using Yahoo messenger and AIM too I think.

Claude