Are you willing to sacrifice everything for high-resolution video? If so, Point Grey's Flea3 USB 3.0 4K camera might just be for you. As the name suggests, the $949 device is capable of recording 4K (8.8-megapixel) video, but what's special is that it comes in at 29mm x 29mm x 30mm — about the size of a large ice cube. Let's not get ahead of ourselves, however: this isn't a RED competitor. Despite the high pixel count, the tiny 1/2.3" Sony Exmor R IMX121 sensor in the unit won't be producing anything near cinema-grade video, and it maxes out at 21fps at full resolution. You're also not going to be able to use any of your lenses, either, thanks to its CS mount. Good thing the Flea3 isn't designed for consumer use, then. The company claims that the (world's smallest) 4K camera is "suitable for a variety of high resolution color applications including automatic optical inspection, ophthalmology, interactive multimedia, and broadcast," and we're sure it'll be a fine option for those who need such resolution in those industrial applications. Still, no one's stopping you if you want to turn it into a needlessly expensive and high-resolution webcam.
Note: Camera in video below is a non-4K version of the Flea3.