Porcupine Trail is a nice connector trail along the White Hill Fire Road near the top of Sir Francis Drake Blvd through White's Hill and Tamarancho.
It is one of the few bike legal singletracks in Marin County Open Space, and part of the Bar Area Ridge Trail. It's a nice, popular trail that features forested climbs and some open meadows. Many people use this trail to start or finish a Tamarancho ride, or longer rides along San Geronimo Ridge Road.
Watch for hikers, especially on weekends. Yield to uphill traffic.