"Windows translation layer" "Run Windows applications on Linux, BSD and Mac OS X."
It doesn't really sound very T-shirt-y to me. I think I'd rather wear a T-shirt with a short, witty slogan than being a walking banner :)
Something like "Releasing your computer of Windows since 1993" or "Closing your Windows since 1993" or "Putting the Win in Windows(TM) since 1993" (okay that last one was more stupid than it should be) would do the work, enough to avoid alcoholic confussions yet not that boring. If someone is that interested in what are the windows you're so interested in closing, they may ask.