B.O.B.'s Ibex trailer ($369; bobgear.com) can handle any forest service road, arroyo, or singletrack you can pedal. The 17-pound chromoly steel trailer easily attaches to the rear 28" wheel of a mountain bike or 700c wheel of a road bike and carries up to 70 pounds of gear, tools, or food. The Ibex was designed to be taken off-road, with an adjustable suspension system to dampen the chatter on washboard and bumps. For an extra $30 you can buy a completely waterproof Dry Sak duffle that quickly fastens into the trailer frame.
If the Ibex is too off-road, try B.O.B.'s lighter and more urbane Yak ($299), which will attach quickly and easily to any bike, making it the natural fit for all you recently converted bike commuters out there.
For light trails or street riding, the Nashbar Cargo Trailer ($200; nashbar.com) is also a good option. Its "Eiffel Tower construction," i.e. craftily engineered triangulated struts, make the trailer strong enough for 45-pound loads, but take note that it's not quite as bomber for the backcountry.