Some more links: Reston Bike Club:
http://www.restonbikeclub.org/
PPTC:
http://www.bikepptc.org/ (cc rider already gave you that)
Civil War Century (Balto Bicycle Club):
http://www.baltobikeclub.org/index.pl/cwc
It's in October, but Seagull Century is a lot of fun (unless remnants of a hurricane pass thru the same day!)
http://www.seagullcentury.org/