Geothermal
I don't know much about geothermal power, but I do know I don't hear much talk about it of late. Anthony Watt thinks this is a mistake, and discusses the potential. To some extent, the problem with geothermal's acceptance is that it breaks our current centralized power model in favor of distributed power. There are few spots where geothermal potential is large enough to run a big power plant, but apparently many where there is the ability to heat a single building.