I have a 3g dongle with limited data allowance so when i'm elsewhere on an unlimited line I download all the clips from youtube i want using Download helper extension on firefox and save them on my flash drive for viewing later.
It couldn't be more simple, install addon: addons.mozilla.org/firefox/downloads/latest/3006/addon-3006-latest.xpi and restart firefox, then whenever you open video in youtube a small icon appears just above the video, press it to immediately download the named video as mp4 wherever your downloads go, or click the side of the icon for more options, like download & convert to other formats etc
It works on most other video sites as well
I use chrome mostly now, and never bother with download managers in this browser or others for various reasons like huge window, need to always run in background etc. In Firefox however there is an exception - DownThemAll is a very simple yet very effective addon. It vastly increases speeds, minimal imnterface with nice features and is at one with firefox - addons.mozilla.org/firefox/downloads/latest/201/addon-201-latest.xpi
Most of you probably thinking "check this n00b trying to tell US about this "i know but thought i'd try and share some info with anyone who might find useful before i request any kind of help