To some, the holy grail of solar thermal is a system without pumps and controls.
Himin of China manufactures a nice one:
At Solaron in the '80's, we purchased the manufacturing rights to a passive DHW design developed by Arthur D. Little. The problem was that the pipes going to the collector would freeze in the winter. Frozen pipes in the attic were so undesirable and difficult to eliminate that we ultimately abandoned the whole project. As far as I know, no one is pursuing this type of design in northern climates.
Now that building codes are beginning to actually recommend insulated roof planes and conditioned attics, the Chinese design looks very interesting. The pipes in the attic won't freeze because the attic is inside the house envelope. As long as the piping to the roof-mounted tank is kept extremely short, it won't freeze either. Natural convection within the pipe and heat conduction along the pipe from the tank and the attic area would prevent freezing.
Note that the water in the evacuated tubes probably won't freeze. Every day they warm up a bit even if the day is completely overcast. This insolation should be enough to prevent freezing even under the worst conditions in Denver.
From Arthur D. Little, we also learned that the roof-mounted tank will never approach freezing temperatures anywhere in the continental US. (Basically there's enough solar insolation even on completely cloudy days to keep the tank above 40 degrees)
The main attraction of this design is that system efficiencies are far superior to pumped designs. Also, with no moving parts to fail, the life-cycle cost is held low, and the ROI is maximized.
Edit, corrections 1/10/11 (the tubes contain water, not heat pipes.)