It's probably superfluous for me to include a link here to this site. If you're enough into mountain biking to find my website, you probably know enough about MTBR. It features what I believe to be the definitive mountain biking forum on the Internet; if it's not, as a whole, the most definitive online resource on mountain biking. A huge selection of trail and product reviews by users is also available. And the classifieds section here must be one of the most active anywhere for mountain biking paraphernalia.
Possibly the most feature- and information-rich mountain bike ride website that's out there right now. The amount of detail and the search and exploration features are as good as anyone can expect. One big strength of this site is that it's not limited to one specific region. The flip side of the site's national scope is that its coverage of a particular local area may occasionally not be as thorough or as hand-picked as that of a site with local focus could be. But, I really wish this site were around when I originally started mountain biking, which it wasn't.
While seemingly structured primarily as the ride log of a rider, this site still contains plenty of reference information on popular Bay Area ride locales. The site's coverage is strongly focused on the Peninsula and the South Bay, though there are notable exceptions. The combined map showing all ride routes represented on the site together is a good tool that should be helpful to those unfamiliar with the area in getting an overall sense of where the attention worthy trail systems are; and this can be downloaded as a KMZ file that you can open in Google Earth on your local computer. The site also features rich photo content for all listed rides.
Mountain Bike Bill
A very rich source of ride descriptions throughout the western United States, and even a handful that are outside the US. Most descriptions are rich with photos, and in some cases even videos. The ride selection featured here for the Bay Area is unfortunately pretty slim, though.
Steve's Mountain Bike Page
A site that's rich in ride descriptions, although it's structured more like a personal ride log (with repeated rides in some locations) rather than a ride directory (where each ride is described only once). There are plenty of map links available. Also features some GPS resources and information on using GPS while/for riding.
Mountain Biking in the San Francisco Bay Area (MTB.LIVE555.COM)
A very wide range of mountain biking resources for a wide range of geographical regions (not limited to the US). The link above is to the page specific to the Bay Area. The ride description selection is very rich, too.
The Ogre's Guide
A staple in the bookmark list of any mountain biker in the western United States. Features a hand-picked list of rides with descriptions from the perspective of an experienced rider. Many ride descriptions features photos and maps, as well as links to other web pages about the same ride or trail, in some cases.
This site is a good resource if you're looking for a list of group rides that you can join. Their list of upcoming rides never seems to be empty. The site as a whole has more of a road riding emphasis, but this link will take you directly to their pages that are specific to mountain biking, which still has a lot of useful information.
This is Garmin's website for logging the GPS data of your outdoor sporting activities online. What makes sites like this useful is the capability to do searches through all the activities of all the users of the site in order find GPS tracks for a particular trail or route in a particular area, of course. The user interface of the site has a clean design and it's quite responsive, though it does experience the occasional hiccup and there are one or two usability issues that deserve to be addressed. You need to have an account in order to search for GPS tracks (which Garmin calls "activities"), but I believe it's free to sign-up.
I'm not at all a fan of the way Strava encourages the wrong type of trail behavior from some riders (many of us must have encountered a rider blasting past us on a singletrack with more attention paid to the effort to attain a KOM ranking than to common courtesy), but it's hard to dispute the fact that this site can serve as a rich source of GPS tracks for mountain biking. Moreover, their Global Heatmap view is probably the best tool currently for eyeballing which biking routes are the most popular within a given region, even if the inability to separate road rides from trail rides somewhat limits its usefulness.
Although this site requires you to create an account before letting you download a GPS track, it's free to create an account. The amount of available content doesn't seem to be too high, at least for the Bay Area, but it's still a valid option to use as one more source to check when you're looking for data for a particular trail or route. However, some of its features (like the interesting-sounding Route Genius for recommending ride routes at a particular location) require a paid membership.
Singletracks Mountain Bike Blog
This professional blog connected to the Singletracks website features frequent mountain bike and equipment reviews, event coverage, technical articles about biking and more generalized biking-related opinion pieces, as well as some how-to articles once in a while.
Diary of Scott Morris
In addition to being the principal author of the TopoFusion mapping application (which I use regularly), Scott Morris is an avid mountain biker and adventurer who gets around quite a bit. His image-rich blog is almost always inspiring, enticing, and interesting to look at.
This one is the biking blog of The San Francisco Chronicle. It has more of an urban/road riding focus, actually. Still, due to the sheer scale of journalistic resources behind it and its coverage of wide-ranging biking issues related to our area (which also impact mountain bikers, frequently), you'd be missing out if you don't at least browse through this blog once in a while.
Passion Trail Bikes
More than just a bike shop, this is an important resource for mountain biking (at least) in the Peninsula. They organize weekly local rides as well as trail work. Their website is a good source of ride information for riding destinations in the shop's vicinity. This shop also features the widest selection of available demo bikes of any shop that I know of in this area.
2Pedal Mountain Biking
A good source for a wide selection of mountain biking maps and guidebooks for the whole nation, including plenty for California. Their California-specific page features a selection of web links as well.
24 Hour Bike Shop
Operators of vending machines stocked with emergency supplies and the essential needs of bikers. Any one of these could be a life-saver if you get in some riding trouble when the stores all closed. They have quite a few machines around the Bay Area strategically located around places popular with mountain bikers, too. You can check the site to see their current locations.
Bay Area Volunteer Information Center
If you're interested in doing volunteer work in trail building or maintenance, this site can be of help to you. While the site, as a whole, is about volunteer opportunities of all kinds in our area, listings that relate to trails and biking are also included among those. Simply use the website's search feature, using "trail" as a keyword, for example. You'll find a number of entries that can be a good starting point.