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I laughed, but actually, wind is being rediscovered for larger vessels:

http://www.plentymag.com/blogs/edge/2008/03/smoother_sailing...

The kite claims a 10-35% reduction in diesel consumption, which is pretty good for a permanent and passive contribution. And you still have the reliability and speed of traditional diesel propulsion.

Or make solar-based weapons against pirates, with a big lens.

You know, I recently helped a friend find a large, but light and flexible, plastic fresnel lens. You can make wood char and smoke instantly and light on fire within seconds, it's really cool.

Check out this wave powered drone: http://www.liquidr.com/

In action: http://www.youtube.com/watch?v=eATawqVOXWI