There are two free rental codes available. One is for McDonalds locations and that code is: SMR8DVD and several other locations have a code for a free movie which is: 9HB6MB22.
I've never really tried Redbox since we have Netflix, but my sister was telling me this past weekend how she loves using Redbox. If you've never used it either, try it with the free code to see how you like it! :)