Google seems to be making a final push to get people buying Chromecasts before Christmas. If you buy one between today and December 21, you'll get a $20 Google Play credit to buy some new movies to stream on your TV. They don't make it too clear, but it appears you will need to buy from the Play Store to redeem this offer. Update: As commenter SEJ326 points out below, Chromecasts from both Amazon and Best Buy are also eligible for the offer.
This offer is in addition to existing offers for 2 free months of Hulu Plus for new accounts and 3 free months of Google Play Music All Access for first-time subscribers. Those of you who purchased a Chromecast before December 7th are out of luck on the $20 credit, though (sorry, Black Friday shoppers). Google is not offering the promotional credit to past purchases.
While the promo says, "Get $20 in movies on Google Play," it looks like you could spend it on any digital content in the Play Store. The offer terms only stipulate that the credit not be used on device purchases. Don't worry if you're planning to give the Chromecast as a gift for Christmas, your recipients have until January 31st, 2015 to redeem the credit. On the other hand, you could just keep the promo credit for yourself and pretend you bought it at Best Buy.
If you are still on the fence about whether or not you want one, you can check out our Chromecast review.