ChambreNoire is an interesting app that can be used to apply a wealth of filters to images. I’ve only played with it for a few minutes and find the interface a bit confusing so I may actually have to look for some instructions on this one. I like it when things are very intuitive right away, but it is currently free (regularly $0.99), so worth a shot. [Download]
I created this image in the few minutes I was playing. Click for larger view.