Chargrilled

Chargrilled is a U.K.-based website selling funny, silly, and off-the-wall t-shirts.  You can submit slogans or designs, and submitted ideas get voted on by the site's visitors.  If Chargrilled uses your slogan or design, you can potentially earn over £100.  Their guidelines aren't very specific, but if you read the terms and conditions, they say that you'll be paid "the value of one full-priced t-shirt" for your accepted idea, and if the shirt with your design or slogan on it sells 30 times in the first 30 days, Chargrilled will pay you an additional £100.