Double Take is an application designed to allow you to quickly compare two photos.
For example, you can use it to reset snooker balls after the dreaded "miss" rule, or restore your amplifier and mixer settings after that last band were onstage!
Double Take overlays a reference photo over your camera while you snap another. Then you can compare the two photos side by side for differences, and slide them across each other to spot subtle differences.
Also useful for tidying up your parents house after your party...