Last week Skype rolled out an application for the iPhone.  If you are on a WiFi network you can make calls to other skype users or phones.  I have used it to make phone calls to land lines a couple of times the quality of the call was really good.  The main problem is that currently you can not make calls through Skype when you are on the 3G network.   Here is an article on the Wall Street Journal about Skype on mobile phones.