UPDATE 2019: All of the below is offline. However, IMDB themselves have similar functionality.
Something I’ve always missed in the Internet Movie Database is the ability to compare two movies. Sometimes I forget an actor’s name, but I do know at least two movies he or she is in. With Stüvel IT’s IMDB Compare, everybody can compare two movies and get a list of actors that star in both. All you need to do is copy & paste two URLs and hit a button! It can’t get simpler than that.