What I am using is the youtube downloader from Sothink, and I am very satisified with the software. There are two editions for the video downloader actually: one is the stand-alone version, which supports many browsers but needs to be paid for the full version, and the other one is the Firefox extension, which only supporrs Firefox but is for free.
Here are the official webpages:
1. Sothink Web Video Downloader Standalone:http://www.sothinkmedia.com/web-video-downloader/
2. Sothink Web Video Downloader for Firefox:http://www.sothinkmedia.com/web-video-downloader-firefox/