If faster heating is needed, the thermosiphon mentioned by DrewE is a nice "free" method, since convection pumps the water.
For the impatient a small bilge pump or aquarium power head can be used, along with 50+ ft of garden hose. Lay the hose in the Sun in a snake pattern, such that both ends can reach the holding tank. Attach one end to the discharge of a submersible pump, and the other end to the edge of the barrel for the water to return.
I use this method with a 32 gallon trash barrel for washing cars, the water is uncomfortably warm within an hour or less.
