Apple have just released a beta release of FaceTime for mac which allows you to call up any iphone 4 or new itouch on a wi-fi connection. It also allows mac to mac calling.
This may also be available on windows too, but am not sure at the minute. It is the beta release and is free to download :)
The direct link is wrong but it wont let me correct it. Use that onehttp://www.apple.com/uk/mac/facetime/