They are Tidal, Wave and River Generation. All three of these are very exciting technologies. There is a forth called Hydro-Thermal but that technology is a ways off as opposed to the others so we will skip that one today.

Tidal generation is currently being used with great success in Europe. Tidal Energy Generation is achieved by using the tidal flow in the ocean to turn a turbine that is capable of generating electricity turning in both directions. This technology unlike wind and solar is totally predictable and consistent day and night as long as the tide is flowing. Wave generation is achieved with special generators that generate from the motion of the waves and of course river flow is self explanatory. We have lots of ocean and lots of rivers. Another way to generate electricity from the ocean are off shore wind farms. If we were to reach a point where we no longer needed the existing oil drilling platforms in the Gulf of Mexico that would be an excellent place for wind turbines and wave generators. Make em' green.

Here are some videos and images in order to get a better understanding of how these systems work.