Sideplayer is a free Chrome extension which opens YouTube videos in a floating player, so you can watch them as you carry on browsing elsewhere.
Once installed, Sideplayer adds its own button to the YouTube console (it's just to the left of "Full screen"). Click this as required and the video opens in a floating window.
Hover your mouse over the Sideplayer window, click Play and the video starts. You can then enter some other URL in the main window, and browse as usual.
The video window may be repositioned by moving your mouse cursor over the middle of the player area (not the top border), clicking and dragging.
It can also be resized, just a little. Tap the Settings icon to the right and you're able to set its opacity, speed, preferred quality and more.
You can't view any other information about the video, skip to the next item in a playlist, or anything similar. But if you click the video title, your main browser window reopens at the YouTube page and you can carry on browsing as usual.
A handy add-on for those times when you're watching YouTube, but really need to be doing something else.