If you want a good downloader for windows then you can look into Free download manager, it's free of cost and has video download capabilities. Here is the link http://www.freedownloadmanager.org/
But if you want a good download manager for linux operating systems like Ubuntu then you can try "youtube-dl". If you want to install it
open terminal/command-line and type
sudo apt-get install youtube-dl
and press enter. After installing it type
in terminal to get documentation on how to use it.
It's a command-line tool, you can download single video or do a batch download and the best part of it is you can download a entire playlist from youtube if you want to. It has download support for other popular sites like Dailymotion, Facebook, Metacafe, Soundcloud.