I use Skype frequently for online games with free conference calling between all the users. The PM facility is perfect for note passing and the file transfer facility great for bunging out maps.
For something more akin to a tabletop then Screen Monkey might be the way to go, but you need a shed-load of bandwidth and need to watch map sizes and refresh rates, because it can slow things down.
Screen Monkey... http://www.nbos.com/products/screenmonkey/screenmonkey.htm