From the FreeStuff.com FAQ page:
HOW CAN I GET SOME FREE STUFF?
Easy. Go to one of our product pages and write a product review (good or bad) of an item you own or have used before. We’ll send you a free TreeStuff shirt. Be sure to include your US mailing address and shirt size in the text of the review. Also, check out our Facebook page for cool ways to win prizes, like this.
This offer will be available for a limited time only.