A friend of mine in Montana has the inground temp heat exchange, but it's hooked up to her well water. It's supposed to cost alot more than a conventional, but pays for it self in 10+ years.
http://www.polarbearwshp.com/?w=PolarBearProducts
It seems like sunfish's, and the fig guy's are basically the same technology, but not the full 6 feet down.
Some pretty simple looking solar water heaters in israel-
http://www.metaefficient.com/wp-cont...rs-on-roof.jpg
I wouldn't mind trying one of those with in floor heating in a greenhouse.