The video must be fully loaded on the player's frame before trying to save it and when saving, add ".flv" to the file name. You will also need a FLV player, VLC is a recommended player.

Just click that and download the video you are watching !

Video Detecter For Internet Explorer! (VideoDetect 0.9.exe )

For FireFox users:
Download helper is yet a simple plugin to download youtube videos.
just make sure to rename the files you are saving to add the extension".flv" and then you can convert the files using a converter or just play them with VLC Player