Free GM Resource: CSUAC
Most of these files were originally posted on the Dundjinni forums, so they are often referred to as the Dundjinni CSUAC. It seems that this huge collection of files bounced around from host to host and ended up here.
When I unzipped the files I noticed that some of the files were dupes, but generally speaking it seemed they were dupes because the originals were corrupted. If you plan on using these files you might want to reorganize the files to mesh with your other art objects, and this will probably take you some time.
Still, these files will probably come in handy if you like to make up your own maps or if you use an online system (MapTools, Roll20, Fantasy Grounds, etc.) to run your game. Just make sure to read the End User License Agreement before diving in.