One of my favourites is the "Pic Grunger." It destroys your photos to look like vintage photographs that have been through WWIII. It's perfect for my Marie Antoinette photoshoot - makes it quite Steam Punk.
Another favourite is the "DragMe Epic" App which allows you to clip art your photograph to make you look like a drag queen.
Here are a few of my experiments.
Pic Grunger
DragMe Epic