ThinkGeek

ThinkGeek sells t-shirts, electronics, and novelty items.  They accept submissions for t-shirt slogan ideas, and if your idea is chosen, you'll receive $250.  Or, you can opt for a $300 ThinkGeek gift certificate if you'd rather have that.  You can read their guidelines on this page.  (You must be logged in to see them, so if you don't have a ThinkGeek account, you'll have to sign up for one -- it's free.)  They won't respond to all submissions, so if you get a response that's probably a good sign.