I'll second MagicJack -- I have three, two permanently installed on machines at home and in my office, and one which I use for travel. Nothing beats being able to call back the US for free, and international from the US are very, very reasonable -- usually a penny or two a minute.

The MagicJack is a small USB device. You can plug a regular telephone into it, or else you can use the microphone and speaker on your computer. It's very reliable and call quality is usually excellent.