Its just a trick to save youtube video from browser cache files..
1.Goto youtube website...
2.Click the video which you want to download...
3.Then play the song....
4.After that goto cache folder
For chrome,
C:\Users\Administrator(Select your user account)\\AppData\Local\Google\Chrome\User Data\Default\Cache
Then right click->sort by->size
(For other browser this location may vary ,just do googling...
5.The files which starts with "f_" is your video file(ex: f_000f11)...
6.Then rename those files with extension .flv(ex: f_000f11.flv)
7.Double click that file to see the video.(flv extension is playable on VLC media player and like...