Parkour Visions is one of the nation's first and finest parkour gyms - a non-profit dedicated to parkour education, community building, and youth fitness. If you're ever in Seattle make sure to stop by.
American Parkour is the largest national Parkour Organization. Their website has lots of great educational material, especially for beginners.
Clips of David Belle, legendary traceur generally recognized as the founder of parkour.
Some short thoughts on the difference, or lack thereof, between parkour and freerunning.