The Chromecast is Google's answer to the Apple TV, or at least some of the features of the Apple TV. The small device makes it easy to stream music, movies, and TV shows from the Play Store, Netflix (NFLX), Hulu Plus, and others on the TV. It works with both Android and iOS devices, and after an initial setup is easy to use from any app.
The feature set of the Chromecast is currently rather limited, but there is a promise of more services supporting the device in the future. And it only costs $35, which is less than half the price of the Apple TV or the Roku 3.