There are some good modern, American ska bands that play a style pretty similar to the original Jamaican sound.
I would put The Slackers from New York at the top of that list. They've been touring and recording since the mid-90s, and have put out some great music. Some other favorites are Chris Murray, the Pietasters, Ryan Scroggins and the Trenchtown Texans, the Stingers, and Westbound Train. Hepcat were also great, but they broke up 5 or 6 years ago.
Another band I really like, the Aggrolites, are actually a reggae band, but they play to the same crowd and often get grouped in with American ska.