Helium Adds Picture-in-Picture Mode to Videos in OS X
Mac: If you want to play a web video while you work, you usually need to resize the browser window and paste it into the corner of the screen. Helium allows you to overlay videos just like in picture-in-picture mode.
Helium can accept videos from YouTube, Hulu, Netflix, Amazon, and just about any other video on the Internet. It uses Safari, so you need plugins installed in Safari to make it work. Otherwise, you just enter the URL of the video and Helium downloads it so you can play it anywhere.
Helium (free) | Github