Its called Hollister and is outside of non other than the town of Hollister California. It is off of the 101 freeway, about 40 miles south of San Jose. Great place to go, trails from very simple to very complex. I believe there is about 5000 acres for 4 wheel vehicles (right next to it is park for motorcycles and quads that is about twice as big). It is owned by the State of California, and the entrance fee is $3 (may be $5 now).
Many people do not realize it, but Pismo Dunes (now Oceanic Dunes) is owned by the state of California, same $3 or $5 entry fee.