Snapseed

Arguably the best app around for enhancing and transforming photos was recently snapped up by Google, but, for now, it remains free and available on iOS, too. For a trusty quick fix, a helpful Auto Correct button spruces up shots amazingly with a single tap, while Selective Adjust and Tune offer more subtle and specific controls. The real fun, though, is in its filters, which include Black & White, Drama, Grunge, and Retrolux effects, along with a Vintage Film look, a Tilt-Shift miniaturization effect, or HDR for landscape shots. Snapseed's swipe-based adjustments, both up and down and side to side on the screen, are some of the most intuitive out there. [Free; Android and iOS]