A 32x32 Tile Set that you can use to create a shadow over a grid with defined edges. The edges are opaque and the insides are somewhat transparent. It is all white based, so you can multiply it with whatever color you want. Also there is a perl script used to generate the whole thing so you can make changes if you want.

The Perl script contains comments on how to figure out what tile to use in what situation. There are 256 diffrent tiles.