BMW Clubs:

BMWCCA - BMW Car Club of America
BayBimmerz - An excellent BMW club in the SF Bay Area
SAMOC - Great club in the Sacramento area. Lots of track gurus here.

Track clubs and organizers:

SCCA (Sports Car Club of America)
Track Masters
UnlimitedLaps
Thunderhill Raceway Park - Northern California

Internet BMW Forums:

Bimmerforums.com
DTM Power.net
Boost Junky.com