Mac OS X: If you're in need of a simple way to monitor audio inputs on your Mac, Apple's Developer Tools includes Audio Monitor to handle that specific task.
While this is certainly a niche tool, there are a bunch of reasons you might want to monitor an audio source. The most obvious is to check levels on a microphone or that another input source is making its way into your Mac. A more specific circumstance would be if you need to pass audio through to your home theatre Mac while displaying another video source (or not). Whatever the case may be, it's one of those handy little tools that's good for both testing and practical applications. In its Developer's Tools, Apple includes the MTCoreAudio package, which comes with a little Audio Monitor application. It lets you set the input and click a Play Through button to toggle whether you can hear the input source through the speakers or your headphones. It has a few adjustments, but basically does just one thing well: monitor audio inputs.