Mac: While it's not easy nor likely for malware to hijack your Mac's microphone or webcam, it's always a theoretical possibility. Oversight is a free app that will alert you any time an app tries to access your camera or mic.
Oversight works pretty much exactly like the previously mentioned Micro Snitch, except that it's free. Any time an application attempts to access your camera or mic, you get a notification asking you to approve or deny access. While you'll see the green light on the webcam pop up anytime you're using it, some malware can theoretically sneak in and record Skype or Hangouts streams that you might not notice, and Oversight can catch that as well. The app was created by security researcher Patrick Wardle, who's made a handful of other lightweight monitoring apps for Mac. While it might sound like you'd need a tinfoil hat to use Oversight, it's still useful even in cases where a normal app does something a little odd, like when the Shazam Mac app kept listening even after you turned it off.