Looks like Voice Over IP is building momentum and I'm starting to mull over whether it's time to try it out. In the US, Vonage looks great while here in the UK, Pipecall looks like a leading contender. However, there are other contenders and resources that should be researched (CallUK, Jeff Pulver, SipPhone, VOIPtalk, Gradwell, Gossiptel). A good website with a UK forum is voip.org.uk (hosted by Gradwell).
To me the biggest attraction is cheap international calls. To call Malaysia, BT charges 42p per minute so we currently use OneTel which charges 8p. But with Pipecall, it drops to less than 2p! Looks promising but how well do they deliver and will it get through the firewall? Don't really have time to figure it out just yet.