The Amazon Echo is a cylinder that contains speakers and microphones that are always listening for you to address it, using the wake word "Alexa." It's designed to hear you from anywhere in the room. It responds with information about the weather, sports, news, and will start playing music when you tell it to. Amazon has a demonstration video, in which they say that it isn't listening until you say "Alexa" -- except that it does have to listen to hear when the wake word has been uttered. The real question is whether Amazon is actually listening, or can listen, to everything being said.
The Amazon Echo is $199, and just $99 for Prime members. It's invite only right now, and you can request an invite at amazon.com/oc/echo.