The overall list of alternate power sources can vary depending on the definition of "alternate" being used. As a general rule, people use the term "alternative energy" to refer to energy sources which may be more desirable for a variety of reasons, including environmental, geopolitical, and financial concerns. Some of the more environmentally friendly choices include hydroelectricity, solar power, and wind farms. Other forms of energy include nuclear energy and biofuels.
There are a number of concerns about alternate power sources including the environment, foreign oil dependence or a combination of these factors. For example, in some contexts, nuclear energy might be considered an alternate energy source. It isn't very common in some parts of the world, and can be cleaner than oil or coal for power plants. Some worry, however, about certain environmental dangers and health risks specifically associated with nuclear plants.
The same could be said for ethanol derived from biological sources, which has some potential benefits in terms of reducing the dependence on fossil based fuels, but can also lead to some of the same environmental dangers. One of the more popular alternate power sources is solar power, which can be gathered in a variety of different ways. Many people add solar panels to their roofs, reducing their power bill drastically, or sometimes totally replacing the need for electricity from a power company.
There are also electrical plants built specifically to gather solar energy, consisting of wide open fields containing many rows of solar panels. One of the challenges with solar energy is that it can only be harnessed during the day. Finding efficient ways to store energy, therefore, is a big priority for many scientists working on solar projects.
Energy from wind and water are both fairly common alternate power sources, and in both cases, the power is captured by harnessing the energy in natural movement. Wind energy is generally harnessed using windmills, and normally they need are placed in areas where the environmental conditions favor a significant amount of wind. Hydroelectricity comes from harnessing the power of moving water in rivers or the forces from ocean waves using turbines, which are often associated with dams. Both sources are somewhat location and conditionally dependent.
Many scientists are excited about the future potential of hydrogen as an alternate energy source. This can be captured from water, so it's virtually infinitely available from a practical perspective, it burns very cleanly, and it has a lot of raw energy potential in terms of pure efficiency. Many devices have already been designed to use hydrogen energy cells, but scientists are working on some technical hurdles that have generally made harvesting of hydrogen a bit too expensive for many day-to-day purposes.