There is a new kid in actioncam town... At least, if the Kickstarter campaign pays off. The "E1" from Z is the smallest 4K ultra-high definition camera with an interchangeable lens system. Apart from the ususl WiFi and Bluetooth connections, the brains are an Ambarella A9 image processor that records 3840x2160 at 30 frames per second with H.264 compression. The relatively huge micro-four-thirds (MFT) sensor from Panasonic (MN34230) allows for amazing low-light performance (up to 102,400 ISO) while maintaining high image quality, up to 6400 ISO. Packed in a body of 75 x 56 x 50mm (w x h x d). The only non-ideal thing is the rolling shutter, but I can live with that... Find some footage here.
Update 2015-03-03: in practice, this camera does not live up to its expectations when used as an action camera. Personally, I don't like the fact that there seems to not be any camera stabilization built in.