Youtube.com and most of the video sharing websites do not allow you to download videos. However these two sites make up for that lack.
- You can download any video hosted on youtube. Just enter the full URL in the text box on home page.
- Other way is to append the word kiss before youtube in your URL Ex: Eg. http://www.kissyoutube.com/watch?v=tRTk0RPXtW4
Kiss you tube does not work for other sites like Google Videos. For these sites you can use
2) http://keepvid.com/ 
Works the same way as 1)
Ankur Jain is a Software Engineer in Test Automation. After a 5 years stint with Accenture and Oracle, he started his eLearning company. A long-time blogger and proud owner of the "Learn" series of websites. 
Yeah! You are right. But this App allows you to download videos in HD quality. Give it a try 🙂
Thanks for the share.
Somehow I don’t like to install unnecessary apps on my desktop. Moreover for things which could be easily performed by web apps. For me kissyoutube.com works the best.
Even you can download directly from your desktop. check out: http://mrintech.com/directly-download-high-definition-youtube-videos-from-your-desktop
Hope you find it useful
– Wishes 🙂