Google's Chromecast has followed a typical arc for a Google product: it was released with a limited-but-promising feature set, and it has since improved slowly but steadily thanks to a constant stream of minor updates. Today, Google finally opened its HDMI dongle all the way up, delivering a public software development kit that should enable any iOS, Android, or Chrome app developer to integrate Chromecast streaming directly. Just as iOS apps can use the AirPlay API to stream to the Apple TV, any developer who wants to can use Google's APIs to stream to Chromecast.
The company also released a list of user experience guidelines and a design checklist detailing some best practices and dictating what a developer must do to earn the "Google Cast Ready" branding. For example, any Google Cast-ready mobile app needs to include a Cast button at the top level of its UI, and that button should always be visible in a consistent location no matter what the user is doing.
To date, the only way to get content from your device to the Chromecast has been to use the Chrome browser or one of the relatively small number of apps that Google has given the green light (Netflix was the first, but others have since been added). Preview APIs were available in July of 2013 for developers who wanted to "experiment," but they were subject to change and not really intended for use in real apps. The new SDK is the first "official" release that's ready for public consumption.
Google has updated its Google Cast extension to version 184.108.40.206 to support the new SDK, and it has integrated the Google Cast Android API into version 4.2 of the Google Play services package for Android devices. Integrating the API into the Google Play Services package should give any device with Android 2.3 or higher the ability to stream to Chromecast using supported apps.