I got this from Fatwallet.com:

Amazon.com is currently offering a free $4 credit towards anything in their Video on Demand catalog, which includes thousands of movies and shows. Enter code AVODGIFT. The credit is good until 11:59p.m. January 3rd, 2010 (PST).

Instructions
1) Click the “Redeem a gift card or promotion code” link below the video player on the item’s detail page.
2) Enter the claim code, AVODGIFT, in the pop-up window and click the Apply button.
3) Click the Buy button to complete your purchase.

Note: If you would like to just add the code to your account but do not want to use it for a specific movie or show at the moment, go to any Video on Demand page and just add the code. For example, go here and click on the button right after “Have a promotion code?” Enter AVODGIFT, then hit the Apply button. You’ll see that your balance now has $4 (or $4 more than what you previously had). Hope this helps.

Enjoy!