VDownloader is a powerful tool that will help you locate and download videos from YouTube, Google Video, Facebook, MegaVideo, and many video sharing sites.
You can use the program alongside a browser. If you're at YouTube and find a clip you like, say, just copy it to the clipboard. VDownloader will detect this, pop up a message, and you can be downloading the video in a couple of clicks.
Alternatively, simply click the search button, enter a key word or two, and VDownloader can list matching videos at YouTube, Google Video, Yahoo! Video or Daily Motion. You can then often play the clips within VDownloader, and download copies at a click, without opening a browser window at all.
The program doesn't just download the source formats, either. Sure, you can have the original FLV files if you like, but VDownloader can also convert the videos to AVI (in various sizes), MPEG, 3GP, MP3 if you just need the soundtrack, and formats suitable for the iPod, iPad, PSP, Nokia N800/ N810, and more.
The authors also produce an enhanced version of the program, VDownloader Plus. This allows you to download videos in high quality or full HS; can schedule downloads; is able to batch download more than 10 files with a single click; is able to convert videos you've already downloaded, and more. It's yours for $29, and you can find out more at the VDownloader site.