DoubleExp (App Store link) is a free double exposure iPhoneography app for your iPhone. Like in the good old days of film, when you accidently reloaded your 35mm roll back in to your camera and ruined your holiday snaps, this app mimics that double exposure effect, but without ruining your pics.

This app has a very simple interface with the options to take photo's, load 2 photo's from the camera roll (1 at a time), apply the effect (FX) and save the resulting image. There are 3 special effects, Half Mix, Screen and Overlay.

I did not have any suitable images to test this app, so I took a couple and applied all 3 effects (individually), but the results of my test did not show any visible differences between them, and as the photo below shows did not really work.

For me this app has limited appeal, as photo's used have to be suited to this effect, but if you have a need for this style, and as this app is free you have nothing to loose.

UPDATE:  It would appear I had been incorrectly loading the images in to this app, and that was the reason the various effects were not working.  I will post an update with photo's soon, along with reviews of other apps from this developer.