Big Huge Labs
I use the fantastic Big Huge Labs site all the time, especially for making mosaics, like this simple one of four of my photos of neon sewing-machine signs in New York. Its easy and fun to work with and they have tons of free options. Big Huge Labs also works with flickr (along with several other sites) for easy photo wizardry.
Some of my other favorite free things to try are generating a color palette from a photo and photobooth-ing four regular old photos into a distinctly vintage look, which I did with my photos from my Kids Yoda Halloween Costume project for CRAFT: last fall! Im also planning to try making a favorite photo into a jigsaw puzzle, which looks very cool.