Here's what I ride in the San Gabriel Mountains, right outside my door, every morning before work. I took these pictures on one of my rare weekday afternoon rides. Since it was took warm to ride hard I decided to take it easy and snap a few pictures for the gallery.
My bike is a '96 Bontrager Race Light running fixed with an ENO eccentric hub. Fork is a White Brothers .8xc with a custom 1" steerer. Front brake is an Avid Mechanical disc with an 185mm Galfer Rotor which really helps me modulate my speed on fast technical descents. Cranks are 175mm ENO which are probably a little on the long side but I haven't had too serious an issue with pedal strike yet. My favorite component is the Jones H-Bar - absolutely the best bars for fixed (or single speed) off-road riding. I'm running 32:17 gearing (~49 gear inches) which seems to be the best compromise for getting me up and, more importantly, down the mountain.