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Google made it official. Chromecast SDK has now been made public inviting Google developers to create their own apps as they wish with one caveat – no porn. This should be no surprise to everyone since they also blocked efforts of people using Google Glass for porn.
If you want to start creating your own Chromecast app, start here: https://developers.google.com/cast/docs/downloads. It will describe what you need to create a Chromecast app. First, you need a sender application and a receiver application. Details are found here https://developers.google.com/cast/docs/developers
For the sender application, obtain files needed to start building your Android, iOS, or Chrome API. For the receiver application, you will need to first determine which media type your application needs to support.
More details can be found at the above links. For users, you can rejoice! Chromecast will be a lot more fun very very soon!