For downloading clips off Youtube, I use these sites, and they don't require plug ins:
http://www.keepvid.com/
http://www.clipnabber.com/
Keepvid works with a good number of sites. You paste the URL they show you into the box, tell it which site you're grabbing from, click on Download, and the page finds the link for you. Then you can rick click and rename the file with a new name and FLV extension.
Clipnabber is good for Dailymotion. Other sites might require other software. I had to temporarily install something for grabbing a clip off Canadian Broadcasting's page.
OTHER STUFF for PCs:
SDP Multimedia has a nice utility for grabbing streaming Windows Media content. There are grabbers for streaming Real Media, but I find that content to be of lower quality.
eRightsoft makes SUPER© and will convert those FLV Flash Video files to other formats, including but not limited to DVD ready VOBs.
And if you don't have VLC, then you're missing out. It can play FLVs without conversion first. DVDs too!
Have fun!!