In San Diego near Mira Mesa
Right off Black Mt Road, between the 56 and Mira Mesa Blvd, there are some softball fields there, the parking for the trail is behind the farmost field from the road.
http://www.mountainbikebill.com/LosPenDelMar.htm
here's a video someone took of there. My friends call it jedi training because your entire body has to move it's way around, below, between the tree branches. Some of the sections are off camber while doing this as well. Very fun stuff, unlike anything I've riden in Socal.
http://www.youtube.com/watch?v=cfeK8bpVoTw