There are dozens of clones of angry birds. Some of them as good or possibly better. And we aren't talking about clones of polished products here, we're talking about a site that admittedly took a 2.5 hours to make. The comparison to a fart app (though their site is vastly more useful, and a unique idea AFAIK) is apt.

Lets take something with a bit less stigma. Background-changing apps. There are, what, 100+ of them? Probably more? Lots of them even share some backgrounds, which is visible if you just skim a dozen from the app store's images. And almost every single one has probably paid back the dev time it took to make, if not through sales then through ads.