XnView Software

I think a really useful feature would be built-in reverse image searching, so that you could initiate a search using Google's search-by-image, TinEye and other similar reverse image search services, right from within XnView's interface. It wouldn't necessarily need to display results in the program; I'm sure this would entail quite a lot of work. Just firing the requests off to the services so that the results open a browser window would be really helpful and save a lot of time.

This could also be used to find duplicates, as well