When people bring up alternative energy, something deep within me compels me to bring up solar energy. I can't help it. I am That Guy. There is something about solar power that just speaks to me; it seems like a clean solution, both in terms of environmental impact and in terms of design.
But I should clarify. When I talk about solar energy, I'm kind of cheating, since I don't really mean photovoltaics. The fact is, a lot of the energy that's readily available on Earth is solar power. Wind? Driven largely by differences in air temperature, which the sun is responsible for. Hydroelectric? Driven by the water cycle, which is powered by the sun. Biofuels? Photosynthesis. (This last one is the most interesting to me - petroleum is a crutch, but if we can engineer plants to produce it (and we can), we can keep leaning on it basically forever, at which point it ceases to be a crutch.)
The fact is, there are a lot of ways to utilize solar power. What's more, it's incredibly abundant, and we know the sun will be around for billions of years. If you're looking at sci-fi scale stuff, the sun is basically the only game in town - nothing else even comes close to it in terms of raw energy output.
Still, for our everyday mundane energy needs, there are a few alternatives to solar power. For example, there's a lot of energy stored in the Earth, in the forms of both radioactive ores (nuclear and geothermal power), and the Earth's magnetic field (it should be possible to get energy from it, but right now that's too exotic to have a name). Both of these are fixed quantities, but they're large fixed quantities, and radioactive metals are relatively easy to get at.
Another energy source is tidal energy, which is actually an interesting case. If you think about it, tides are generated by the interaction of the Earth and the Moon, so tidal energy is actually borrowing energy from the Earth/Moon system somehow. Unfortunately, I can't quite figure out where the energy is coming from. My gut tells me that it's coming from the Earth's rotation, but my understanding of how tides work is kind of tenuous, so I'm not sure. Anybody wanna back me up on this?