audiohijack will do it
vixy.net is web based and works when it feels like it
tubetv will do it as well
All are freeware, well audiohijack is trial I think but the trial doesn't run out and does what you want it to.
I think audiohijack is probably best just purely for doing that, as in quickest and easiest, very versatile for audio, but for general youtube shenanigans tubetv is an excellent app.