(but only if you have a flatbed scanner)

Print resolution is usually 300 - 600 dpi (or more, depending), and screen resolution is 75 dpi. You can exploit this fact to make some darn nifty background images for your desktop.

How? Take interesting-looking things, like pretty leaves or orange peels or blocks of wood or leaves of grass, and scan them in at 300 or 600 dpi (you can go higher, but I've found that it tends to not come out so nice-looking, at least on my scanner, and isn't worth the extra disk space) -- for most things, you shouldn't worry, but if you are worried about the scanner getting dirty, you could put some plastic wrap down first. Once you've scanned them in, now a leaf that is only a few inches in size comes out as 1800x1800 or so pixels, which for most people is about twice the size of the desktop. (If you really want to do something cool, take a bunch of leaves, some of them turned different colors and some of them green, and scan them all in). You can crop the image to whatever looks coolest, and play around with color balance and brightness to your heart's content (things also sometimes look really interesting if you reverse the colors or shift them around).

Anyway, now you've got (depending on the size of your monitor) a 15-or-so inch leaf godzilla sitting on your desktop, which is darn nifty. And some things don't even look like they did when you scanned them in. One thing to do is to scan in a tangerine or orange peel, and presto-chango, you have what looks to most people like a massive pool of bright orange bubbling goo oozing across your desktop.

Or you can be boring and stick with the pretty colored leaves... its all up to you.