Here is a crappy, unfiltered photo.
And here is that same photo with a variety of weird and wonderful filters applied.
I tested these all on an iPhone 4; most are also available for Android and tablets. They vary in cost from free to a couple of dollars, and I only included ones that loaded relatively quickly, were intuitive to use, and allowed me to use pictures I already had on my phone (rather than requiring me to take a photo in-app).
3. Toon Camera
4. Color Splash
Another app that was basically made for landscapes, this allows you to play around with light and shapes in a totally new way. You can overlay one of the many pre-made filters or make one of your own. (A note, though, that if you want to edit your photo for contrast, brightness, etc., it's probably best to do that beforehand in a different app before applying one of these guys — this is more focused on the aftereffects.)