Linux only: Video downloading utility UTube Ripper saves a copy of your favourite YouTube videos for your offline viewing pleasure, even converting them into a non-flash format.
Installation on Debian-based versions of Linux like Ubuntu is a breeze—just download the .deb file from the downloads page and double-click to install everything. You'll want to make sure that you have Adobe Flash and the ffmpeg library installed as well, or the utility won't end up working properly.
Once those steps are completed, you can launch the utility, paste in a link to a YouTube video and click the Download button to retrieve a copy. The Convert FLV section of the application will let you convert into MPG or AVI format, or you can use the "Rip audio only" option to convert a YouTube video into an MP3 file.