Microsoft's Kinect, the always-on 3D camera/mic that could have been used to read people's facial expressions while watching ads. I'm glad it failed.

Google speakers which have a microphone array they use to measure the shape and size of your room. Definitely only for acoustic purposes of course...

And some people pay money for that, which makes me fear this might become standard in all kinds of devices. Those things aren't even that great in terms of features. In movies like "I, robot" the machine overlords were at least useful for cooking.