This is supremely nerdy. Back away from the computer now if you're not up to the task. Are the normals gone yet? Good. Okay, if you've seen Inception, you know that it has a complex plot (see this plot diagram for more on that). Spoiler alert: the plot involves a series of dreams; dreamers may bring others along into a dream; and there are many dreams-within-dreams. It's crucial to know who's the current dreamer at any given time to understand what's going on in the film. And this is explained in the film, but it's also sorta mind-bending (I honestly didn't follow the details of it when I saw the movie in the theater, and had to watch it carefully later to figure out how it worked).
In the following video, Chris Baker uses Mac OS X's Finder and a series of nested folders to explain the dreams-within-dreams. This is the perfect example of something that's awesome to a nerd who liked this movie and knows what Mac OS X is and completely baffling to the other 98% of the population. Enjoy, fellow nerds. (Note: you may want to watch it on Vimeo and/or go fullscreen to see the details.)