xVideoServiceThief is a tool for downloading your favourite video clips from a lot of video websites. xVideoServiceThief also provide you the ability to convert each video in most popular formats: AVI, MPEG1, MPEG2, WMV, MP4, 3GP, MP3 file formats.
Cross browser userscript to download videos from YouTube in various formats and sizes available.
Find size of videos before downloading (http://pradip.x10.mx/index.php/download-details/36/Helper)
Check Readme.txt in Files section for installation instructions.