OS X: While there are plenty of free utilities out there to record audio, few let you record system audio without having to jump through hoops. Piezo simplifies the process with a one-click method to record audio from any application.
You can set Piezo to record from any open application, including web browsers, Skype, and even Spotify. This might come in handy if you're looking to record Skype or Google Voice calls on the fly, or if you need to capture audio from a webapp. Piezo doesn't come with a lot of recording options and you can't adjust levels or tinker with the recorded audio, but its ease of use will come in handy if you're looking to make a quick recording.
You can download a demo version from developer Rogue Amoeba's site that allows up to 10 minutes of recording per session or you can purchase the full version for $10.