Portal goes on my list of "Best $20 I ever spent on a computer game" right behind Morrowind. Sometimes, I'd be talking with a client, and the conversation would turn to gaming. Portal defies explanation. So I took an old 40 GB hard drive, put it in a USB case I wasn't using, and followed a step-by-step guide to "unleashing" the game. Now, instead of trying to tell someone how the game works, I can just show them. (Just a little demo, though. I've since replaced the full copy with the "First Slice" version.) The "etc" folder contains the Steam installer.
I dressed up the partition a little, the idea being that the drive was stolen from Aperture Labs by someone who escaped.
Custom icon, custom backdrop, custom description (warning message) and custom desktop.ini to make them show up.