Raspberry Pi camera module has pretty good characteristics and with software support you can do lots of interesting stuff. This is OK until you need more than one camera. What if you need more than one camera? There can be few solutions to problem – use another Pi per camera and join them with some webserver, use USB based webcams or try thing differently and multiplex multiple cameras to this single CSI connector. IVPort selected third option where he build camera interface multiplex module.
Multiplex module is designed as stack-able, so you can connect 4 cameras with single module and 3 GPIO control pins, 8 cameras with two modules that require 5 control pins and max of 16 cameras with 9 control pins. Of course cameras aren’t displaying all at once. Through software or manually you can select which camera to turn on while other are off. This solution is great for monitoring things around, building self aware robot platforms when cameras are stacked in cube. Other uses might be for building bullet time camera setup, because switching time between them is about 50ns.