MacTubes - YouTube video Player and Downloader for the Mac.
I had been a fairly faithful user of keepvid.com, using it anytime clients sent me videos that would need to be edited into a reel or otherwise repurposed. But after the big Java security scare of 2012, keepvid never seems to work correctly. So I decided to do some googling and found MacTubes. Likely, it's a secret because the web page is mostly in Japanese, but the app itself works great.
It's a free download.