MacTubes is a great way to watch a bunch of YouTube videos on your Mac. You can set up playlists, search and of course watch the videos. The only downside is that there’s no way to log in to your account and play from your own playlists, but hey—it’s free!
Work great, and much better than keeping a browser window open on your second monitor. I typically have Minecraft “Let’s Play” videos by Paul Soares jr. playing on mine. Not sure why, but it’s nice relaxing background entertainment for working late night.