What are your favorite no-cost marketing campaigns for a product launch?

I plan to do a "ReTweet in the next 24 hours for a chance to win a free [my product]", so yes, there's a "cost" in that I don't get profit from the work/product for the 1-3 winners, but I'm not counting that kind of cost for this question. What are some of your other favorite no-cost marketing campaigns?