http://flavio.tordini.org/minitube
This app let me watch youtube videos without using a browser, perfect if you are doing something else (like programming) and watching a video.
You can also achieve the same thing with totem native ubuntu video player but you have to download an extra plugin in order to watch videos with a higher quality:
http://www.simplehelp.net/2008/05/23/how-to-watch-youtube-videos-in-the-ubuntu-media-player/
http://www.omgubuntu.co.uk/2012/09/desktop-youtube-app-minitube-adds-search-filtering-youtube-fix
http://flavio.tordini.org/musictube