Bear Valley - There are many miles of mountain biking trails in Bear Valley, the ride was a point to point ride with lots of technical rocky sections and a really killer slickrock downhill.
Eagle Creek - An extremely challenging downhill run which ends in the town of Dardanelle. Optionally it can be run as a monster loop starting and ending on Clarks Fork Road (Milo's Run).
New Melones Lake (Glory Hole) - Glory Hole is one of my favorite 'local' trails, local being within about 45 minutes drive of Stockton. It's located just outside of Angels Camp on Lake Melones. There are about 20 miles of good hard packed singletrack suitable for beginning riders.
Coyote Point (Area 51) - The Coyote Point Trail is located at Lake Hogan near Valley Springs. It is also called Area 51 because it looks like the landscape is from another planet with harsh looking rocks, barren ground, and strange plant life.
Bear Creek (Hogan Dam Road) - Located on the back side of Lake Hogan this is a short singletrack trail connected to several miles of rocky dirt road and cow trails. The nice part is that the road is miles long and almost noone comes out here.
Lyon's Lake - Just east of Twain Hart is a small lake called Lyon's Lake. There are a bunch of trails accessable from here and the other trailheads which connect to the old Railroad grade which runs past the lake. There are about 30 miles of Rails-to-Trails class RR grades and plenty of miles of Singletrack if you know where to find them.
Parrot's Ferry Downhill - Also known as the Mine Trail this trail is located right next to the Parrots Ferry Bridge. Mostly this is a downhill fiends hangout. This is a great place to go if you are looking for huge jumps, stunts, and big rocks to launch off of.
Penn Mine Trails - A small OHV park/ trail system in the town of Campo Seco (just north of Valley Springs).
Pinball - Just east of Twain Hart is a huge area filled with all kinds of trails. This one called Pinball is rough and rocky experience. Big parts of the trail are made of some sort of volcanic rock which tears up riders and bikes alike. This is a tough trail with a unique apeal.
Pinecrest Peak/ Hammill Canyon - A hidden cache of incredible Sierra Nevada singletrack right off of highway 108. A 15+ mile downhill run which can be shuttled or ridden as an all day epic. If you ride the climb this ride is more than 30 miles with 4500' of climbing.
Red Hills - I imagine mountain biking on Mars would look much like this place. This place is filled with steep rocky red hills and really fun single track. Try to be nice to the horse folks, they built this place.
Strawberry Flume - Beginning and Intermediate riders will love this 10 or 17 mile double loop of fairly flat but fun singletrack and RR grade.