This entry is a Bike Hacks Classic.
As much as we would all love a product to work immediately as advertised, this is not always the case. A great example are bike fenders. The whole point is to keep road splash off of you. Thus, the fenders should be as close to the tires as possible. The bigger the gap, the more splash.
Ken from 650B Wheeled Bicycles left a comment on Astoria Bike's blog about using wine corks to fix an annoying gap between the rear fender and the down tube. Astoria Bike put the hack to the test with Grade A results and a simply smashing tutorial. Visit the full entry and get drinking!
Another great fender hack related to mounting a fender on a bike that is not equipped for fenders can be found here.