OverSight is a free security application that monitors your computer's webcam and microphone. Harmful software installed on your computer without your knowledge -- called "malware" -- can activate these devices to violate your privacy. This app simply keeps track of times when the camera and microphone become active. If the devices are turning on when you're not around, or when you don't tell them to, it could be a sign that your computer has malware running on it.
Using the app
OverSight not only lets you know when the camera and microphone are active, it lets you know which process is activating the camera. You can choose to allow or block particular processes, letting legitimate applications through and stymieing attempts to hijack your hardware. This allows you to detect malware that "piggybacks" on legitimate uses of the camera, such as tools that activate when you make a video call and allow a third party to intercept it. The app's interface is simple and easy to use, although as a free project it isn't exactly graphically polished.
Basic protection for video and audio hardware
OverSight is a free tool, and it's not perfect. Some forms of malware which gain more privileged access to the computer's operation could possibly evade its protections. Still, it's a valuable free tool for protecting your computer's security from unauthorized use.