Feel free to add your Drupal drive MTB site to this Wiki!
IMBA Sites
IMBA
IMBA Canada Events
IMBA Affiliate Sites
Folsom-Auburn Trail Riders Action Coalition
Northwest Trail Alliance
Dallas Off-Road Bicycle Association (DORBA)
North Shore Mountain Bike Association (North Vancouver, BC, Canada)
Other MTB Related Sites
2pedal
Trailforks
See Global Riders1 for a list of sites running on this platform. Notably NoBMoB - Northern Beaches Mountain Biking Group and Perth Mountain BIke Club.
1 Global Riders is a Drupal based multi-site implementation which offers similar features to all sites running on the same platform and shares content between partner sites as appropriate.