I personally like the google reverse image search hack :).
I used to just run that python script manually on my computer. Now I can just call it from my front-end js when users upload images and return back some text about the image.
But you can do just about anything. Users have done r statistics from js. Or python sentiment analyses (lots of good libraries there) whenever they get user comments in their rails app.
I imagine they have an undocumented api for reverse image search. The team at Google has released this extension[1] for adding reverse image search to the context menu, which is currently the only extension that enables reverse image search for images only accessible while logged in (think if you want to reverse image search a preview of an image someone emailed to you).
I was thinking about intercepting the requests it makes and making an open source extension that does the same thing, but I haven't had the time yet.