It looks like a thumb drive but plugs into HDMI.
During the event, Google also rolled out Chromecast, a $35 HDMI dongle for getting web video and other digital content to TVs. The thumb-drive-sized device uses a single-core processor running a slimmed down version of Chrome, along with an 802.11n controller.
So far, Google Play, Netflix, and YouTube apps support Chromecast, and Pandora support is in the works. Google launched a developer's kit for building support into a variety of other devices and apps.
A Chromecast feature that's still in beta will send data from a computer screen directly to a TV. This would be particularly useful for viewing photos.
I've seen many approaches to linking mobile devices and web content to TV screens. However, I have not seen any approach that's so cheap or works so well across Android, iOS, Mac OS, and Windows systems.