DetachVideo is a lightweight application that, when running, allows you to pop out windows for YouTube and other sites' video content.
When the mouse cursor is over a video, hitting F12 (this can be changed) pops the video into its own window. The size is dependent on the original source - DetachVideo can't resize video unfortunately.
The pop-out window is set to always be on top of other windows, so you can have a video playing while you work on something else without losing sight of it.
It is a little buggy, and doesn't work on all video sites, like Vimeo, for example, and if you navigate away from the source page, you'll lose the video, which is a pain. DetachVideo is a tiny .EXE file, which needs no installation, and can be run from a USB portable drive without problems.
DetachVideo is a pretty cool idea, though it needs a little work to become the finished article. It works perfectly with YouTube, so it's certainly useful.