Open Street Map
[Jul 11, 2009; Outdoor]
Google Maps really blows when it comes to finding trails to mountain bike on. If its not made for cars, its not on the map. You might get lucky and see some indication of trails when using the satellite view, but thats a painful way of planning a tour.
Surprisingly, Open Steet Map has all the trails and more landscape detail than its commercial counterpart. So don't be fooled by its unprofessional looking UI, it is your friend off the road.