besides already famous Keepvid.com, Gassyerotica recommends "Video Download 2.0" .
it saves flash videos from Youtube, Google Video, Pornotube, Grinvi... and many other sites with simply one click...
First of all, you must use Mozilla Firefox .
You can download it here
Firefox is the most fastest web browser I know, also , Gassyerotica is best viewed by that browser.
Start to use it if you use slow Internet Explorer!
After you get it, download addon called "Video Download 2.0" here
Shortly, when you are on youtube, just visit the video you want, by the flash video choose right click with your mouse and choose download video.
Further notes are available on mentioned page....
Easy and fast! ::thumbsup