Woot is pretty darn popular. They offer one deal a day across their spectrum of sites - Woot, T-Shirt Woot, Wine Woot and a bunch of others.
Shirt Woot puts one shirt up for sale each day as well as runs a themed derby. The derby shirts on Shirt Woot are submissions for which people vote. Usually, the top three vote getters get made into shirts, but, for the three's a crowd derby, they only made the top two.