Last year, Redbox gave away a free rental code every Monday. After it became a huge success, they changed the program to one free code per month. They then changed the program once again and instead of releasing one code for everybody, they asked people to sign up on their site and get a text message with a unique, free code.
So while I'd love to share with you the code that I get on the first Monday of each month, I can't as it's only valid for me. So if you have a Redbox nearby and want to make sure you get a free rental code each month, make sure you sign up!