All right, that was an obvious one. Sometimes you just have to pick the low-hanging fruit.
So, photo-mosaics! So very over, until you can make them yourself, and then they're pretty cool! I used the step-by-step instructions offered by David Louis Edelman on his blog to make this mosaic of me out of the book covers in my LibraryThing catalog. Click thru to Flickr and do "All sizes" - these things are so cool when you can zoom in.
I also did some great ones of the kids out of the family pictures, but they're like 75 megs, and I subscribe to nothing that allows me to upload a file of that size to the web.
God, I am just dripping with hair, aren't I? I've had it cut since then.