I'm not sure how often this actually happens, but there's speculation that hackers can use your computer's webcam to watch you, including the FaceTime camera on Macs. The simplest solution is to cover the camera with a sticker. However, that's not ideal for aesthetic reasons. Plus, it could leave residue behind.
SpiShutter has a self-titled solution that lets you physically block the embedded webcam your Mac. It has an opening in the center of the strip that lets you enable your webcam by sliding the strip left or right, and then blocking the camera when not in use.
SpiShutter is designed to stay on your computer at all times, even when the lid is closed on a MacBook Pro, MacBook Air, or MacBook. The low-friction powder coating is designed to prevent any scratches to the screen and leave no residue.
However, I couldn't get my SpiShutter to stay attached. It kept falling off, which sort of defeats the whole purpose. For that reason, I can't recommend it.
However, if you want to give it a try, SpiShutter retails for $19.99 and is available now on Amazon and SpiShutter.com in three colors — black, silver, and yellow.