FotoMix is an interesting tool that allows you to combine two photos to add, remove or mix elements from each.
This might allow you change the background on an image, for instance. You can take a person from one image, and add them to another (or just remove them altogether). Or you can create photo montages or collages in just a few clicks.
You can, of course, do this in photo editors anyway, as long as you've a little knowledge about layers. And if you understand how these work then FotoMix will seem rather basic, and limited in the extreme.
If your image editing skills aren't so great, though, FotoMix could be useful, as it allows you to create genuinely impressive effects with very little knowledge or effort.