Also, there's
www.jajah.com. It lets you make calls, phone to phone, for free. You need a computer to initiate the call from their site, but then your phone rings and your call is on the phone (no need for headphones/mic).
Last time I used it, they allowed 30 minutes of free calls a day. For more than that, there's a fee, but it's pretty cheap all in all. They also let you create a button/link that you can send out to people or put up on your website. Then people just click on your button to call your phone from theirs. Pretty nifty and useful for businesses.