Use these codes to get a FREE rental at Redbox kiosks (not valid online). You may use each one time per credit/debit card.
9MXW4LPC (exp. unknown)
NW2PCLP3 (exp. Dec. 5th, 2013)
You will need to swipe your credit/debit card, but will not be charged unless you return it late (after 9pm the next night). In that case, you'd pay $1.29/day for each day it's late.
Check here for Redbox locations. You can use each code once per credit/debit card.
Other Current Codes: (as far as I know - let us know in the comments how they work for you!)
DVDATWAG (valid only at Walgreens locations)
DVDKROG (valid only at Kroger locations)
DRIVEIN (valid only at Sonic locations)
You can also sign up at their website or text "Redbox" to 727272 to join the Redbox text club and get free rental codes sent to you! (messaging and data rates may apply....yada yada)