Here are the free voice chat services I've tried for long-distance calls:
MSN Messenger
Yahoo Messenger
Skype
Google Talk
Gizmo
I'd say Skype is the best in terms of voice quality and it's user-friendly. The problem with Yahoo and MSN Messengers is that sometimes I couldn't obtain a voice token or sign in.
Skype quality is pretty good - both to another Skype account (free) and to a phone/mobile (not free of charge though). I've also tried MSN Messenger talk+webcam conversation - but the quality was rather poor. (This might be due to low bandwidth though)