You can get $6 in free Google Play credit for each Chromecast device that you have on your network.
Just visit the Chromecast Offers promotional page to search your network for your device and any available offers.
After you check, if you have more than 1 Chromecast device, you can refresh the page and check your other devices for additional $6 credits.
The $6 credits can be used toward Google Play purchases.
A special thank you goes out to Android Police for posting about this deal.