Good new stuff:
venturacountytrails.org
corbamtb.com

General State Information

• Pete Fagerlin. Pete is a great guy with tons of videos and pics from rides all over, but mostly his home state: California.
• Team Big Bear
• California State Parks (many of which are MTB-friendly)
• U.S. National Forests (Pacific Southwest region)
• "Cycle California!" magazine (including upcoming off-road events)
• Per Löwdin illustrates a visit to several local mountain biking areas
• Biking Destinations in and around Southern California
• maps for Turlock, Tahoe, Downieville, Santa Rosa, San Jose, and more I should probably add links to the individual maps from the areas.

General Northern California Information

• OgreHut.com The Ogre's Guide to Mountain Biking Northern California. Great trail descriptions and maps. Mostly Northern Cali, but good southern info as well as forays into central Oregon and Moab.
• Shasta-Cascade Mountain Biking 40 Top Trails in Northern California's National Forests
• A nice set of NorCal trails from Anne & Russ

San Francisco

• San Francisco Mountain Biking A website to preserve and promote responsible mountain biking in San Francisco through advocacy, trail stewardship and community outreach.
• Responsible Organized Mountain Pedalers (ROMP) SF and South Bay

General Southern California Information

• Desert Mud Southern California Mountain Biking
• MTB Madness
• Southern California Mountain Biking
• Epic Riders MTB Club - southern California
• Cuyamaca to Noble Canyon in Julian, California is on IMBA's list of epic trails
• A handful of SoCal trails from Anne & Russ

Bay Area

• Bay Area Rides is a great looking new site that covers the entire bay with good detail.

East Bay Area

• Bicycle Trails Council of the East Bay
• Bicycle Trails Council of the East Bay - San Francisco Bay area
• Delta Pedalers
• East Bay Area - Ogre Hut
  Close in to Oakland, Pleasanton, and Discovery Bay there are miles of trails open to mountain bikers. Single track is rare but there are plenty of high quality trails in the East Bay area.
• Briones Mafia - Briones East Bay Regional Park

North Bay Area

• Keith's Bike Page
• North Bay Area - Ogre Hut
  Areas to the North of the San Francisco Bay including the Napa and Calistoga areas.

Marin

• Marin Trails Mountain Bike Page
• Trips for Kids (Marin and nationwide)

Bakersfield/Southern Sierras

• Southern Sierra Fat Tire Association
• Kern County - Ogre Hut
  Many trails are located in the hills east of Bakersfield and in the mountains near Lake Isabelle.
• South Sierra Nevada - Ogre Hut

  Lake Hogan, New Melones Resevoir, and Redhills are the main riding areas in the southern portion of the Sierra Nevada Mountains.

Fresno

• Spatdog Mountain Biking Adventures

Lake Tahoe/Downieville/Nevada County/Sacramento/Central Sierras

• El Dorado National Forest (California) - This site covers the areas to the south and west of Lake Tahoe. It covers basic information you'll need to know when you're planning a trip, but it's not as complete as the Tahoe site.
• Mountain Biking Nevada County - Nice list of trails with details and small maps.
• BONC (Bicyclists of Nevada County) MTB community with trailguide, advocacy, and more.
• Outside Inn a motel that caters to Mountain Bikers (Nevada City)
• Downieville Outfitters (530) 289-0155 Full service mountain bike shop. Rentals. Downhill shuttles. Guides. Much much more. Home of the #1 downhill in America!
• Tour of Nevada City Bike Shop (530) 265-2187 - For hardtail and full suspension bike rentals in Nevada City. Also, a full selection of bike sales including: VooDoo Cycles, Marin, Specialized, Schwinn and Bianchi.
• Lake Tahoe Area - Ogre Hut
  World famous for beautiful sights, gambling, and the crystal clear lake, the Lake Tahoe Area is a chosen destination for many outdoor sports. There are many great Mountain Biking trails in the Tahoe Basin for all skill levels
• The Trail Page has descriptions of trails in Sacramento and Tahoe.
• Sports and Recreation - Folsom Bike Trails Folsom Recreation Bike Map With Plenty of bike trails to choose from Folsom has a path for all levels of riding. These are bike paths, not trails.
• Iron Horse.us GPS mountain bike trail guide with Sacramento and Tahoe GPS trail routes and a world directory of MTB sites with GPS data and a review of GPS cartography software.
• Lake Tahoe Mountain Biking Guide
• Fish's Mountain Bike Trails
• Folsom-Auburn Trail Riders
• Folsom Lake/ Auburn Area - Ogre Hut
  Centered around the Folsom Lake and Auburn the Central Sierra Nevada area has miles premium singletrack only a few miles east of Sacramento.
• North Sierra Nevada - Ogre Hut
  Downieville, Bullards Bar, South Yuba Trail, and many, many miles of less know but excellent singletrack in the Downieville/ Nevada City Area.
• Downieville, California is one of the few areas cool enough to make GORP's list of Meccas Beyond Moab
• Downieville Outfitters - Downieville
• Mountain and River Adventures - Kernville
• The Flume Trail - Lake Tahoe
• Yuba Expeditions - Downieville
• The Big Boulder Trail in Downieville, California is on IMBA's list of epic trails
• South Yuba in Nevada City, California is on IMBA's list of epic trails

Los Angeles/Orange County/Santa Barbara

• Orange County's Hottest Mountain Bike Trails
• Mountain Biking in Orange County (archive.org archive)
• Pasadena Mountain Bike Club
• Hollywood Pro Bicycles
• Buckhorn Loop in Santa Barbara, California is on IMBA's list of epic trails

“The Bicycle Kitchen is a cooperatively-run bicycle Culture and repair space dedicated to improving the lives of Angelinos through environmental justice, cultural awareness, community and youth outreach using the bicycle as it's tool. We offer what no other bike shop can offer in the greater Los Angeles area: A safe space for cyclists from all walks of life to repair their bicycles, meet other cyclists, enrich their lives through bicycle awareness and find new outlets for bicycle culture.”

Santa Cruz/Monterey/Hollister/South Bay Area

• Mountain Bike Rides in Los Angeles (mainly on fire roads)
• Henry W. Coe State Park: official site, trail info, more info from an old site, mountain biking in Henry Coe
• Hollister trail map
• Santa Clara Mountain Bike Trails
• South Bay Area - Ogre Hut
  The combination of coastal forests, cool ocean breezes, and great singletrack make the South Bay Area a great place for mountain biking.
• Henry Coe in Morgan Hill, California is on IMBA's list of epic trails
• Mountain Bikers of Santa Cruz (MBOSC)

Mountain Biking in Palo Alto Open Space Areas

• Skyline Ridge Open Space Preserve
• Russian Ridge Open Space Preserve
• Windy Hill Open Space Preserve
• Monte Bello Open Space Preserve
• Arastradero Preserve Palo Alto

Santa Monica

• Mountain Biking in the Santa Monica Mountains (archive.org archive) (is it still live?)

San Diego

• San Diego Mountain Biking Association
• Important Update on Del Mar Mesa Trails!

Central Coast

Central Coast - Ogre Hut. The Morro Bay Area

The following links are not organized in any way:

Maps from trogspace.com:

Crandall OHV Map
Crandall and More
Pinecrest Peak
Strawberry
Dodge Ridge
Milo's Run
Hammel Canyon

• BikeMonkey.net - Sonoma County
• FATRAC - Folsom-Auburn area
• Fish's Mountain Bike Trails - Nevada, Sierra, and Placer counties
• Foo Riders.com - mostly southern California
• Mere Mortals Mountain Biking - a laid-back group who bike anywhere in the Bay Area
• MotionBased - detailed GPS data for most rides in California, as well as elsewhere
• MORCA - Monterey Off Road Cycling Association
• Mountain Biking - Bay Area - greater San Francisco Bay
• MTBBill's Trail Reviews and Pictures - southern California & elsewhere
• Mtbguru - a directory of GPS data and pictures, most in California 
• NorCal Mtn Bike Racing - Northern California Mountain Bike Racing News
• NorCAMBA - Northern California Mountain Biking Association
• Over the Bars Mountain Bike Club - southern California
• RockMtb - San Diego & vicinity
• San Diego Mountain Biking - southern California
• San Diego Mountain Biking Association - southern California
• San Francisco Mountain Biking Page - greater San Francisco Bay
• Singletrack Mind - southern California
• SocalTrailRiders.org - southern California
• T.R.O.G.S. - Trail Riders of Greater Stanislaus (County)
• Foo Riders video from southern California
• RockMtb - video from southern California
• San Diego and Vicinity Mountain Biking
• SocalTrailRiders videos from southern California
• Winston's Video Archive - southern California

• The Bay's Best Bike Rides
• Bicycling on Catalina Island
• BikeCal.com Cycling guide to Northern California
• @LA Recreation: Bicycling in Southern California
• Los Angeles Area Bike Paths
• Mountain Bike Rides in LA
• Mountain Bike Trails in Northern California
• Mountain Biking in Death Valley National Park
• Mountain Biking in Southern California
• Orange County's Hottest Mountain Bike Trails
• Palo Alto Area Rides courtesy of Palo Alto Bicycles
• Sacramento Valley Cycling
• San Diego Mountain Biking
• San Francisco Bay Area Bike Rides
• Santa Clarita Bicycling & Hiking Trail Information
• Wine Country Cycling Resource

tahoe trails

http://arnica.csustan.edu/mtbike/maps.htm
Turlock, Tahoe, Downieville, Santa Rosa, San Jose, and more
i should probably link to the whole page, and make individual links to the maps from the areas

California