Prism (U.S. $2.99) takes a unique spin on applying photo filters. You load an image, and the app checks to see what colors make up your photo. It then displays a filter wheel, and as you move around the outer edge and explore the central parts of the wheel you will see a variety of effects. Some are subtle, others are pretty wacky. Of course, the ultimate filter selection is up to you. Prism also lets you engage the iPhone camera and snap away with filters applied.
There isn't a help menu, but you can view how it all works in an online video. I would put the app in the "you figure it out by exploring" category. There's nothing wrong with that, but some people might be frustrated. Even selecting photos isn't exactly standard. When you start, the latest item in your camera roll appears. You can swipe to the next one, or use the wheel to rapidly move in your library.