Hawaiian shirts are great. Wear them in summer, winter, onstage, off, on the pull or on the piss.
Mambo make very loud ones, not for the faint hearted.
Here are a few of my favorites:
Blue and White. I've pulled in this one, so its a favourite.
Cream and Blue. Tasteful, one for any occasion.
Red and White. This one goes to eleven.
Green and Yellow. This one is huge, good for the summer.
Mambo shirt. As worn by Scorpius on Farscape.