I just thought that this might be a thread worth starting. I don't have anything worthy of a full tutorial, but someone might find this useful... A good way of getting the painted look that Valve got for their textures is to make a pallet for yourself by making a new image, then taking the general range of colors you want in any given stroke, and hitting the render/Clouds filter. then, take the clone tool to it with a rough brush like the one I used here. drop the opacity down to about 75% and paint patches of any area you want, while offsetting occasionally. You can see the results I got. Lets hear some other ideas!