How do you save video files off youtube.com?

To extract songs from videos on Youtube, I use this http://www.downloadhelper.net/. It is an add on for FF and then I use http://www.aoamedia.com/audioextractor.htm to convert the files to MP3. Both are very very easy to use and the extractor is very fast.

I have a question though, is this legal? I use it at home often, but my university (NCST) is very strict on illegal downloading so I don't want to get caught.

I did a not so thorough search on the matter and found nothing. I say since the videos are already available online to the public, then there shouldn't be a problem with wanting to view them offline. I say continue to download off youtube until it is specified by youtube or your school that it's not allowed.