OK, I just know my friends, brothers, neighbor, kid, and mainly my wife are all going to literally kill me for this but I just couldn't help myself!
I just love the iPhone's "entertainment" apps (well, the few ones which are not crappy) that allows tweaking images, faces, etc, but FatBooth is probably the funniest one for me so far. It lets you grab a picture, mark the face and supersize it in a few seconds.
The great thing about this app, is the friendly UI which is not just intuitive but also fun using. A good UI is always something that can make me promote an app.