Picamerge (U.S. $0.99 on sale) is a rather unique photo app for iOS that is designed solely to create double exposures to make some unique looking images. Gee, in the old days when we had double exposures with a film camera, it was usually by accident!
Picamerge lets you use photos from your camera roll and albums, or you can take pictures using the apps built-in camera functionality. When you have your double image, a slider bar across the bottom lets you adjust the blend, and filters along the bottom of the screen let you try some preset unique looks. The filters vary the transparency of the images, and can add some color effects if desired.
Getting a worthwhile double exposure can take some planning, and it's not the kind of effect you will use very often, but it's great to have when you want it. I can foresee creating some very nice holiday photo cards with a snow effect or candle overlay.