The Raspberry Pi cam can be used as a webcam by the Pi or another computer connected to the Pi via USB.
But why? That's like wanting an open source hardware mouse – look up the datasheet of the main IC and the suggested design is pretty much the entire schematic.
I actually have an open source mouse - the ploopy. I love how I can easily 3d print parts to repair it but mainly its just comfortable as all hell. Similar idea for the webcam with the addition of privacy.