Variable neutral density filters are very useful for photographers that like manual control in bright light. Instead of paying hundreds of dollars for one, though, you can make your own from just a few cheap polarising filters instead.
A variable neutral density filter filters the amount of light coming into the camera, making long exposures, depth of field, and other effects easier to do. They can usually be pretty expensive, though, despite the fact that you can get the same effect with a few cheap polarisers.
There isn't a ton of advanced DIY involved here, just a bit of screwing and unscrewing (albeit with very small screws). You just need to take apart one of the filters, flip the lens, reassemble it, and screw the two together to make one normal filter, with the flipped lens on the outside. You'll then be able to rotate the front filter to adjust how much light you want to let into the lens. Hit the link for the full instructions, complete with photos and tips for using it effectively. And, of course, be sure to head on back here and share yours in the comments.
Simple Variable Neutral Density Filter [Instructables]