RedBox is a great source for cheap rentals (just $1 a day), but it can also be a great source for free rentals. Sign up to receive their weekly newsletter, and you'll get a free rental code on the spot, as well as additional rental codes tucked into the newsletter from time to time.
Bonus Tip: Watch Walgreen's weekly sales ads for free rental code promotions. They've run several "buy-this-item-and-get-a-free-code" deals lately.
Snag even more free rentals from Blockbuster Express, a newcomer to the kiosk scene. Just sign up for their New Release newsletter to get your first free movie code. Then, watch for more codes to appear in future issues of the newsletter.
Bonus Tip:You can find more codes on their website. They usually run in the top right corner of the homepage.