1. KeepVid - It supports almost all popular sites like YouTube, Metacafe, etc. Have bookmarklet option to download videos in one-click. You have to rename downloaded file to add .flv extension. You can use VLC media player to play flv files.
2. Flashload - Replica of Keepvid, but also available in Deutsche language. Supports 50 video sites. Maintains list of last 20 videos downloaded on homepage.
3. videodownloadx - Same like keepvid but have option to convert video online into 12 different formats. They ask for email address in conversion option. In case you have concern about it try next..
4. vixy.net - Supports less number of site than keepvid and less number of conversion formats than videodownloadx but simplest to use. Also they don’t ask for your email address. Read More…
5. Media Converter - Supports many video sites. Conversion options gives nice compression ratio. It have too many options and wizards. Good for advance users, bad if your looking for quickie. Converter is also available for download as windows only version.
6. Zamzar - It also feature reach like Media Convertor but instead of giving their convertor away they offers online tool where you can upload existing flv videos for conversion. Good for Linux & Mac users as they don’t have desktop software to do the same.
7. kcoolonline - List of 211 video site they supports is eye-catcher. They do have a convertor but no bookmarklet option.
8. TubeG - Supports search and download from YouTube. Flagged video is available for download. Have bookmarklet as well as many interesting section to browse.
9. ilikeyoutube - Ability to search and download videos using top tags. As name suggest its only for YouTube.
10. YouTubia - YouTube only. Offers keyword based as well as user name based search option.
11. Video Download Helper - It is second most popular firefox addon with 234,815+ downloads. Do I need to say more about it?
12. Video Ook - Supports less number of site but better customization options. Most important feature - file renaming after video title.
13. GreaseMonkey Script - Those who love to do it GreaseMonkey way can use this script. Its for YouTube only. It adds a direct download link on all YouTube pages as shown in image.
14. ByWiFi - You can search, watch & download video from YouTube & Metacafe. Registration is optional but useful to track you usage history across site. It will be great if they add support for more video site.
15. Orbit Downloader - Integrates nicely with many video sharing sites. Just hover the mouse on video and a "Grab It" button will pop up immediately! Available in 30+ languages.
16. Vdownloader - It can download and convert videos from many video sharing sites, saving them in AVI or MPG format.
17. KeepV - It also supports download as well as conversion. Conversion to MP4 format may catch iPod users. You need to manually copy-paste video URLs - Bad integration! Requires Microsoft .NET Framework.
18. Youtube Grabber - Very lightweight, but works for YouTube only as its name suggests!
For windows we got many softwares but most of them were paid. I don’t see any reason to pay for things when there are better alternative in freewares.
19. Youtube-dl - This is a shell script. After installing you can use command like youtube-dl URL to download and play video automatically. Videos played using VLC player.
Mac OS Users
20. GetTube - Only option I found for Mac OS. Only youtube is supported as you might have guessed it. Good thing supports conversion to MP4, MOV and AVI formats.
Additionally script mentioned for Linux users can be used from terminal.
21. Youtube Rippin - Finally if you are a coder and this large list impressed you, you can think of starting a site using this PHP script. Moreover if you have a PHP interpreter on your system, you can run this locally.
Source: Devils workshop.
The BEST tip i encountered recently is :
just add "pwn" before the youtube URL and hit enter it gives the download links for the file to download..
Eg: www.youtube.com/abcdef... change it to www.pwnyoutube.com/abcdef and you get download links . :)