Hydroelectric Power
Hydroelectric power is probably the most promising of all the alternative fuel sources. Actually it is probably wrong to call hydroelectricity an alternative fuel source given that it is one of our oldest sources of electricity and currently accounts for about a fifth of the world's total electricity generation. Nevertheless there are a number of advantages that hydroelectricity has over other means of generating electricity so it is very likely that it will become much more widely used in the future. That being said despite the many advantages of hydroelectricity there is a fair amount of opposition to its continued use.
The greatest benefit of hydroelectric power is that it is a renewable source of energy. In fact it is by far the most widely used renewable energy source in the world. The power is generated by using the flow of water to spin a turbine, so long as there is water available hydroelectric power will be a useful form of energy. There is also the significant advantage that hydroelectricity produces no emissions. Since nothing gets burned there are no greenhouse gases being released.
The other big advantage that hydroelectricity has is that it can be produced at a relatively low cost, at least competitive with other source of electricity. While there is a high capital cost of building a hydroelectric plant the operating costs are very low. This is something that is also true of solar and wind power. However unlike solar and wind power the reliability of hydroelectric power makes them cost effective. Solar and wind power really aren't cost effective right now because of the fact that the generate power on an intermittent basis that means that you either need to save the excess power when it is available or find an alternative source of power when power isn't being generated. This significantly increases the cost. These aren't issues with hydroelectric power since electricity is produced on a consistent basis. There is also the fact that hydroelectric plants can last for decades once they are built. This means they can be amortized over a much longer period than wind farms or solar farms where nobody is really sure how long they will last.
Despite all of the benefits of hydroelectric power there is a fair amount of opposition to them. The main reason is the effect they have on the local ecosystem and in some cases the human population. Hydroelectric power usually involves building a dam, which in turn floods a large area behind the dam. This makes the land unusable and will have an effect on local wildlife. In some cases it has even required that large populations of people be moved. There is also the issue of the effect that the dams have on fish. This is particularly true of the salmon in North America who travel from the ocean up river to spawn. Obviously having a dam blocking the way is going to be a problem. Solving these issues will be important if hydroelectric power is to continue as a major source of energy.