Redbox has released a new FREE rental code that has been reported to work at Kroger and Kroger affiliate stores! Just enter DVDKROG during check-out to get your free rental. I do not have an expiration date on this code, so use it soon!
Some have even reported being able to use this multiple times per credit card/e-mail address. This is not usually how Redbox codes work, so I would count it as YMMV.
Also, if you are brand new to Red Box you can also try the codes DVDONME, BREAKROOM or DVDATWAG (valid only at Walgreens) to get free rentals. These codes are good once per credit card/e-mail address.