If you use Google's Chromecast device, you can get $6 in free Google Play credits. It's just for US owners at this time, but it seems like Google is trying to encourage more people to use their Chromecast for renting movies, books and more from the Play store.
Either way, it's easy to redeem:
- Open the Chromcast app
- Select your device and "Check for Offers"
- Confirm your serial number, and you're good to go!