YouTube Middleman is a light-weighted software written in Java used to download youtube videos in any number with the download manager you desire.
For instance, you have the youtube link :
http://www.youtube.com/watch?v=jQ_ExkfcBao
Add the prefix ( http://localhost:9090/? ) & make it like this :
http://localhost:9090/?www.youtube.com/watch?v=jQ_ExkfcBao
Now YTMiddleman will act as a middleman who gets the page link, process it, and streams video-file to the requester.
Here's more details :
Simple Steps to Download A YouTube Link
Getting help from the YT Middleman to download a list of videos
An Overview of settings and Choosing video quality
Wiki: An Overview Of Settings
Wiki: Link Helper
Wiki: Simple Steps To Download A Link