Here's a very cool way to make free* VOIP calls from computer-to-computer and even computer-to-landline:
http://www.voipbuster.com/en/index.html
*"To counter the misuse of our network and to reserve capacity for our genuine testers, we have limited our Free-Calls policy to 1 minute per call. If you want to enjoy UNLIMITED free calls, just register and buy 1 euro worth of credit."
I'm using it and it works very well via headset.