Sun, fossil fuel and tidal currents for example
Electricity
Yes indeed it can be. Any source of energy is potentially dangerous to some degree or another, but it can depend upon how extreme or absurd a situation you might want to imagine. Ultra-violet light...
No
I don't know all of them and that's what I want to know also, but here's 3 that I know. There's Coal, Oil (Gasoline), and Natural Gas.