Extension page: srnyx.com/sywf
Simple YouTube Windowed Fullscreen is an extremely simple extension (only 12 lines of code) that allows you to open YouTube videos in their own pop-up windows.
This is great if you're watching a long video or movie and want to multi-task. Or, you just like being able to see your taskbar and to easily manage the YouTube video's display.
This was made in a few hours (had to learn a bit of JavaScript and Chrome API), but I'd be happy to add any new features and to fix any bugs!