Just came across this interactive map where you can show the states you've visited. There's one for the world, too, but the only foreign countries I've been to are Canada, Mexico and France, so I don't see the point.
However, I've been to quite a few states in the USA:
visited 26 states (52%)
Create your own visited map of The United States
Though not as many as I'd thought. I see the East and the South are foreign to me yet...