Windows/Mac/Linux: Movie library managers usually cost upwards of $30, and they're often only available on Windows. TinyMediaManager solves these problems by being free, cross-platform and easy to use.
The app allows you to add movies scattered across your hard drive(s) into one central library. It can pull metadata from a variety of online sources and, of course, you can edit it as you choose. You can even compile specific movie sets. The obvious application is for trilogies of movies, but you can even create your own sets, so, for example, you can have the entire Marvel Cinematic Universe in one place.