Here is the new code I received from Redbox. Good for one free rental today only (expires at midnight on 12/17). Use promotion code: 6K3FS8 when checking out!
If you want to get these codes yourself, sign up for their email or text messaging services. Each Monday they send out a code for a free rental. You can sign up through their homepage.
Looks like they have added some holiday rentals too! Enjoy!