This will be a quick one. If you get movies at Redbox (those big red machines at grocery stores and drug stores) you should know about how to get them for free :)
You can use the codes below when you check out the movie - just press "Enter Code". You still need a credit card but it will not be charged unless you keep the movie for more than one day. Each of these codes will only work once per credit card:

BREAKROOM
DVDONME
DVD4ME


This website has more codes, but the three above I have tried many times without fail. Enjoy!