Southern California Trailhead Map
Well this was cool while it lasted! Being a crowd sourced and publicly managed resource it was at-risk of douchbags and fat-fingered types screwing it up. Many of the trailheads have been erronously moved. I now highly recommend using the trailforks apps. I'm leaving the rest of this page up as-is. There are some good knowledge nuggets here but you will need to verify the 411 from a second source.
You can scroll around here and find a trailhead near you. Some of these trailheads have links here to the site as well as other places. This map was generously created by Dave Walsh who has set it up as a collaborative effort of many mountainbikers. The contents of the map are growing everyday so you will need to zoom and scroll into your area of interest to see all of the content. While I can not attest to its complete accurancy, it is a great user-friendly resource.
For you geeks and curious map users out there, the interactive map above is not a direct link to the Google hosted map. The map above is from a datafed KML file hosted here that allows the users to avoid having to click between the mulitple pages of the Google Map. It typical can take up to three hours for new entries on the Google Map to fed into the map above. I'll also have to recreate the feed everytime the map grows to another page. When I last checked there were five pages of trailhead information on the Google Map.