Skype is a paradigm-changing, VOiP client that enables you to have a conversation over your PC for free, anywhere in the world. All you need is a computer with speakers (or headphones), a microphone and a broadband connection.
I just tried Skype this morning for the first time, and after some technical difficulty with my rarely-used microphone, I had a conversation with our friends in Dublin. The clarity was excellent, definitely equal to that of a long-distance phone call and with no latency. There was occasionally a slight bit of interference, but otherwise I was really impressed by the service.
At the moment, both parties have to have Skype running on their PCs to talk. In the near future, Skype is launching a service that enables a Skype user to call somebody who has a regular phone.