FloatPlayer is a simple Mac app that plays a floating YouTube-video on your screen. The player is visible even on top of full-screen apps. It's also completely open source. Check it out on GitHub.
@hellotom I had never heard of either, but I suppose it's not very different. (It is actually quite a funny coincidence that the screenshot on Helium's page also displays the video on top of Xcode viewing the apps source code.)
Fuzion
Origami
FloatPlayer