PhotoTropedelic: Image processing apps are a dime a dozen, so in order to matter, a single-filter app has to be pret-tay, pret-ray, pret-tay cool. The trick here is that PhotoTropedelic doesn't just run a straight, predictable filter; it interprets sections of photographs and applies different filters to each. Instead of a mishmash of colors, you get stripes, stars, and other designs. Plus you can export to scalable PDF, which you'll probably want to do after seeing what kind of results you get. At $2, though, the price is a little steep.