Radiant Design Institute
Though not an appealing website(clunky UI) has a lot of really well-grounded and useful information I believe can be very useful especially to do-it-yourselfers.
Boiler Stoves UK
Seems like a sister website to Stoves Online, which specifically explains how boiler stoves work and can be incorporated into a smart and efficient heating system.
Stoves Online UK
A great resource for learning about the different elements that make up a heating system with a very rich offering of solutions if you happen to live in the UK.
US Department of Energy – Geothermal Heat Pumps
A useful section from the US Department of Energy website explaining Geothermal Heating