xkcd Goodies - Compiling
A favorite around the ThinkGeek World Domination HQ, Compiling effectively explains to your boss what you're doing on either xkcd's site our ours when your boss walks by.
Based on xkcd's Compiling comic, this shirt increases your programming and swordfighting skills to 18.
Note: The first decoration the Code Monkeys + our token BOFH undertook when we got to the new office was to abscond with the projector, blow this comic up huge on a piece of foam core, and take some markers to it. And a fine job they did, indeed, while their code was compiling.
"I'm not slacking off. My code's compiling." on the front with office swordfighting on the back in white on a black 100% t-shirt.