Absolutely love Redbox. They're mostly in grocery stores around here (Boston), but I know they are elsewhere in the country for sure. This website sends out an email each week with the latest promo code good for one free rental.
http://www.insideredbox.com/
You have to register with them by giving your email address, but I haven't gotten any spam at all (from them at least). Every Monday, you'll receive the URL to this week's code. Very nice. Either way, it's only a buck to rent. Definitely worth it.