Ever wanted to add a filter to only part of your image and not have to worry about selections or masking?
Well the answer is to use a combination of snapshots, and the history brush.
To do this simply open an image, and make sure that it is flattened if it isn't then flatten it. Next apply a filter to the image, then goto the History Palette and at the bottom of the palette click on the button to create a snapshot, this will create a flattened image thumbnail below the first one in the history palette, to the left of that second thumbnail click the empty square (this determines where the history info will come from when painting), then in the history undo the filter (but the snapshot will stay, which is what you want). Then go back to the layers palette and select the history brush and start painting, you will be painting from the snapshot (which will be a non existant image that had the filter applied to it on that snapshot that you took) and you will get the filtered image showing whereever you paint with the history brush.
If you screw up part of the image and paint where you didn't want to, don't panic, simply goto the history palette and click the empty square to the left of the first thumbnail, go back to the layers palette and with the history brush paint back over top of the filtered part until satisfied, then if you wish to continue with more filtered painting then check the other snapshot in the history palette.