I have been using Moyea YT downloader. It's very good. The free version saves the file as flv. There's a 'for purchase' version that will download and convert to numerus other formats. I already have converters that I like so I use the free version.
Will tell you a simple procedure. Nothing to install. Just watch the video to end. Press CNTRL+U, you will get the source code of the site. Then press CNTRL+F, this will make the find option open at the bottom side. At the space write .flv. Then you will get the exact url of the video. You will get the video downloaded on the next second.
Good tip. You can also try https://addons.mozilla.org/en-US/firefox/addon/3006/ for firefox or another popular software is http://www.dvdvideosoft.com/products/dvd/Free-YouTube-Download.htm. Do you simply want to download videos and that's it?
Check Leawo Free YouTube Downloader. It can download YouTube videos to your computer as FLV format, besides, it can download almost all videos from the Internet.
Go to http://www.youtube.com Type in the search area, any video that you would like to see. Click the video so you start watching it. As soon as you're on the page to watch the video, go to the Address bar(where you typed in www.youtube.com) and just erase the Youtube word, while substituting it for the word Voobys. Download the video. It will ask you to name the file, so type in any name you wish to name it except add either .wmv or .avi or any other format you wish to convert the video as, at the end of it.
I use DW helper plugin for firefox, sometimes i also use keepvid.com But you can use various youtube downloader... just googling it and you will get a bunch of results
I use Internet Download Manager It can also download videos from Facebook, or any other flash video source