Got a DirectTV dish you don't need? Make a powerful fresnel lens to heat stuff up quickly!
http://www.youtube.com/watch?v=Q90i31JIQ3M
Reading about using a
water mass to keep greenhouses warm makes me think this is an easy way to heat up some water. I'm not sure on the management of the water and the dish and how long it could be left unattended, but this seems like a very simple and powerful use of the sun!