Crazy, hidden trail in Oakland Hills. The climb is a fire road (see directions). You could also hitch a ride to the top near Merritt College and do the single track/down hill portion (some folks do this with friends and alternate drivers).
The downhill portion has some _very_ technical sections, some jumps, a graffiti covered cement relic of a bygone era, dips, swerves, some huge drop ins and at the end, if you are daring enough, a red bull style drop down off of a huge, old cement tram anchor (the big brown thing at the end).
The fire road is a nice, quick exercise or even pass through to skyline. You could come down the fire road too if you're not a fan of the epic down hill.