I’ve learned a very simple trick for downloading youtube videos. I explain it using an example:

• I’d like to download this youtube video: http://www.youtube.com/watch?v=4PcL6-mjRNk
• I add “mp4” to the above address, after the word youtube. It will be: http://www.youtubemp4.com/watch?v=4PcL6-mjRNk
• Now I open this address. The resulting site gives me a link for downloading the video. Its format is mp4 and it can be downloaded in multiple sessions (resumable).