Chrome: The picture-in-picture mode used by the YouTube app for iOS and Android is a great way to continue browsing on YouTube while a video is playing. The YouTube Picture in Picture extension adds that functionality to Chrome on the desktop.
The extension works pretty much exactly as you'd expect. Search for a video, right-click the link, then select the Picture in Picture option to start playing it in a small window in the corner. Or, click up a video, then click the back button to automatically drop it into PiP mode.
Sadly, the second you leave YouTube the video will stop playing, but it's pretty useful if you want to add the same functionality that the mobile apps have to Chrome.