RedBox is a great source for cheap rentals (just $1.20 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.
2. The LibraryHave you checked out the movie collection at your library recently? If not, you could be missing out on a great freebie. Think: new-release movies and no rental charge.
3. HuluSurf over to Hulu.com, and watch one of the movies in their collection. Some require a Hulu Plus membership, but many are free.
4. Cereal Boxes
General Mills and Kellogg's both run free movie promotions from time to time. Purchase specially-marked boxes. Then, redeem the codes for free digital copies of movies.