While I'm a huge fan of Threadless, they only come out with new designs once a week. Those wanting a daily fix for their artsy t-shirt needs should check out TeeFury, Woot Shirt, and Design by Humans. All three accept artist submissions and let user votes decide which tees should be printed. TeeFury and Woot Shirt are the most economical options, with shirts from $10-11 shipped.
Me? I'll check 'em out each day but probably hold out for the next Threadless $5 sale. They have a tongue-in-cheek vibe that suits me to a ... well, you get the idea.