Freeware open-source
get-flash-videos (Linux, Windows and Mac OS X) allows to download YouTube videos without installing Adobe Flash Player. It is handy for saving videos for watching offline. You don't need to install and/or upgrade flash player if you use get-flash-videos.
get-flash-videos support not only YouTube, but also eHow, Brightcove (used by many sites like Channel 4, Daily Telegraph ...), BBC (news, etc), Metacafe, 5min, Google, fliqz, nicovideo, vimeo, Blip, Break, Collegehumor, Muzu, Sevenload, Megavideo, Wat.tv.flv
See also:
FLV players