Annadel - This is one of mountain bikings classic destinations. Located just east of Santa Rosa along highway 12 this is a great trail system that seems taylor made for mountain bikers.
Boggs Mountain - Boggs Mountain State Forest is located just south of Clear Lake along highway 175. This is a big park filled with miles and miles of phenomenal buffed singletrack. It is located a bit out of the way for most people so you generally don't run into a lot of people on the trails either.
Mountain Biking at Point Reyes - A beautiful vacation destination with mostly wide multi-use trails which can be enjoyed by the whole family. Plus one hidden gem which offers a bit of a challenge.
Oat Hill Mine Road (Calistoga) - One of the most unique and rewarding mountain biking trails near the Bay Area. This trail has a rich history and a challenging nature which makes it one of the most unique and sought after trails in the area.
Rockville - A Mountain bike playground just outside of Suisun City. Lots of rocky singletrack packed into a small park.
Skyline Park - A playground for mountain bikers. This park isn't the biggest trail system around but it offers plenty of technical riding for the more skilled mountain bikers.