I LOVE Threadless tees. Not only do I have friends that work there, but their designs are from independent graphic artists with a incredible sense of style. AND they actively engage their customers in choosing designs, and participating in company events. Actually, I'm kind of jealous of the friends that work there. I think it would be a great company to be a part of.
Also: they have spectacular sales just before Xmas. Just about everyone I know gets a unique tshirt at the holidays.
In addition to great shirts, they also have fantastic contests. Apparently, they have a contest to create cakes from their shirt designs. One fellow blogger made a "Crazy Cake" that mirrors one of my favorite shirts. Major kudos to the Crazy Cake Company for the delicious re-creation!