Who Stalks My Cam is a free application which can alert you when processes access your webcam, save these events to a log, or block access according to your own rules.
By default the program just displays a warning. This can be tweaked in various ways - display time reduced, the "alert" sound disabled - or you can turn it off entirely.
A simple "Webcam Events" log lists all recent webcam accesses, including the process and parent process responsible, the start and end date, and the duration.
There's an option to build a whitelist of applications which have permission to use the webcam, and block access to everything else.
You can alternatively apply more general rules. For example, if the webcam is accessed by a background process, you can tell Who Stalks My Cam to close it again, or to terminate the process.
Who Stalks My Cam is a handy webcam monitor with some useful extra features. The developer also says it will soon watch for microphone access, key logging and other spying actions-- we'll be interested to see how it progresses.