Textures are always one of those things that if used tactfully and well, add a nice touch to a project. For instance, that weathered-looking note looks nice when you’ve got it on weathered paper. Or maybe you’d like some grass in the background of a website or flyer for a fresh, green and inviting look.
It’s fun to photograph your own textures (like that hardwood floor that’s just the right color), but making textures can be just as fun. And they’re a good way to experiment with brushes and layers and layer modes.
There’s a fantastic tutorial about making easy grunge textures over on BittBox, and I used it to make this:

Kind of reminds me of a grungy, greasy window in an abandonded building. Or some broken-down building found in one of those shooter games my brother plays on the Xbox.
If you like the texture, you can download it here.
The BittBox tutorial has some great tips, and you can also download some hi-res brushes there if you don’t have any watercolor brushes or grungy ones. And again, there are some tips about using layer modes, and this is a good way to try and see what effects the different modes have.
Speaking of layer modes, there’s a handy article on just that topic over on Fudge Graphics. It’s a nice visual reference (we’re all mostly visual folks after all, right?).