"The Superimpose app (.99) was first brought to my attention from Abby Escobal via a twitter conversation over the summer.
It does a few cool thing on an iPad that are very needed for layering images, creating overlays, and silhouettes. Photoshop-ish apps like ArtStudio do these things too, but it is 4.99 and a complicated interface. This app has some limiting features, but you can do so much still that it's a MUST for my elementary students. I spent a ton of time yesterday and today exploring possibilities and LOVE what I've discovered."