Okay, it's not a costume so much as it is a cheap helmet made from a cardboard box. But it's mine, it's nerdy, and it's something I'm strangely proud of.
Out of the five games that came with The Orange Box, Portal turned out to be the surprise hit. Sure, lots of people looked forward to Team Fortress 2 and Half-Life 2 Episode 2, but Valve's crazy little space-warping puzzle-adventure managed to receive the most praise from even the most jaded reviewers (not-work-safe language in link). It was short, but chock-full of clever, challenging gameplay and remarkably dark humor.
One of the most memorable bits of the game is the appearance of the Weighted Companion Cube. Half-furniture, half-NPC, the Weighted Companion Cube was your best friend in the game. I won't spoil what happened, except to say that... sniff... it was just such a brave little geometric shape.
The Weighted Companion Cube, like Portal itself, turned out to be a surprisingly popular little doo-dad spawning nerdy efforts ranging from pillows to cakes. While I lack the artistic and craftsmanship skills necessary to make a genuine, accurate Weighted Companion Cube, I pushed my meager Illustrator and glue stick abilities to the limit to create a Weighted Companion Cube costume. Yes, it's justa cardboard box with paper bits glued to it, but I like to think it keeps the spirit of the Weighted Companion Cube.
Happy Halloween, everybody. This is my costume, the William Companion Cube. Check just below for a larger pic.