WireTap Anywhere is a professional audio-capture utility that can take input from any source and redirect it to any audio-recording application. That means you can take audio from your entire system, a hardware input, or an app like Skype, and then take that directly into GarageBand, Peak, or any other recording app.
You manage WireTap Anywhere through your System Preferences, and you can create and save any number of "virtual microphones," which show up almost as if they were hardware. There are other apps (including open-source ones) that can help you with this kind of redirection, but WireTap Anywhere excels with its reliability, easy interface, and ability to aggregate any combination of software and hardware inputs into a single virtual device. Though it might not be the best value, WireTap Anywhere is a solid solution to a specific problem.