Here is an interesting study about different energy sources and how economical they are. It is a hard to read graph, because there are different ways to implement each technology. Each "mode" is given a darker stripe.
The cheapest energy by far is geothermal. So if you live near a volcano, you need to drill for steam. It is even a reason to move next door to volcano -- although periodic lava flows and explosive eruptions are a problem.
As we all know hydropower from dams is cheap power too. Surprisingly a related water-power technology, ocean tides are not cost-effective. It seems the tidal energy systems need to be big, heavy and expensive.
Burning wood or farm debris for heat is cheap too; this is wood gasification and then running a regular internal combustion engine on that.
Using the sun to produce hot water for your house, also makes sense. Using it to make electricity is pretty expensive. Solar power is best used to make heat not electricity.
All this comes from the UN Climate Change group called the Intergovernmental Panel on Climate Change.