Jordan Golson has a nice content comparison chart of Apple TV, Google’s Chromecast, and Roku.
The new Chromecast, which costs just $35, plugs into an HDMI port on a television, and needs to be charged via a USB connection. The Roku box is perhaps the most direct competitor for the Apple TV, with the Apple TV supporting AirPlay mirroring and streaming from iOS devices while the Roku delivers a number of additional content sources.
It’s a mixed bag, really. No YouTube for Roku, no Amazon Prime for either Chromecast or Apple TV. Integration to iTunes, a Mac, and AirPlay make Apple TV more robust, but there’s no DVR capability, and limited and restricted apps.