There are, I believe, 18 countries you can ring with the international add-on. From the add-on page:
400 International Long Distance minutes
Included countries: Canada, China, France, Germany, Greece, Hong Kong, India, Italy, South Korea, Malaysia, Mexico, Poland, Portugal, Puerto Rico, Singapore, Spain, Thailand, United Kingdom, United States, and Venezuela.
You can also use it for US calling if you like. If you call the States frequently, but not enough to warrant switching to the $50 plan, the international long-distance add on works out to a dollar cheaper than the US/Canada one for 400 minutes.
