Even in our city limits (and obviously without a mountain), mountain biking happens. Of course, things are better with a mountain in the mix. Visit the Bicycle Rides page to find a mountain near you.
There is a new local club called Yolo United Mountain-Bikers. The Cal Aggie Cycling Team also encompasses mountain biking. There is also a high school mountain bike team, the Davis Mountain Bike Team, racing in the NorCal High School Mountain Bike League.