If you've stumbled on someone else's YouTube playlist of great music videos, movie clips, or other videos that you enjoy, you don't have to rebuild it yourself to ensure you have a permanent record of it. Copy YouTube Playlists is a webapp that will do the trick quickly and automatically.
The tool was built by Amit Agarwal of Digital Inspiration, and he shows you how to use it in the video above. He notes that YouTube used to give you a way to copy another user's playlists, but that has been removed, so this tool will do it for you instead. You have to log in with your YouTube account (so the app can get permissions to create playlists on your behalf), and then you can paste in any playlist URL you want to duplicate. Set the privacy of your new playlist, and you're done.
This will save a new copy of the playlist, but it won't help if a video in that playlist is deleted. Hit the link below to give the webapp a whirl.