Well, as my geek code says I can dress up, but most of the time it's jeans and t-shirts. Occasionaly shirts of the banded collar and long sleeve variety. I once worked for a computer money laundering scheme that was part of a bank. For that I had to wear tie, slacks, etc... They didn't like the chainmail tie either.
I would say my favorite shirt was one with a programmer at his computer with "Work or DIE Project" on screen with the caption "I work best with a gun to my head" Same artist has a shirt with a caffiene bell curve with the the top being "See God" and then the end of the curve having "Triple Shotgun Murder"

I worked for one company that distributed a T-shirt with "*!$# Symantec" on it. (We were being sued by them)