Voipdiscount. Very low rates, and free to many countries (land lines).

It's like a phone card: you call the access number, enter your pin, then the number you're calling. You can register numbers so you don't have to use a pin.

I use them from home phone and cell phone, and have called to many international locations.

They also have a web-based front end and a downloadable version, and those can be used computer-to-phone, or call back (a-la Google voice).