Question: I am watching some video lectures on You Tube (licensed under Creative Commons - Don't know the version but I have right to copy).. So, after watching (streaming), how to download the buffered content..

I guess it would be on Browser Cache! but how to get it on HDD?

Answer: 
You can try `youtube-dl` - a python-script.

<http://youtube-dl.org/>

If you use a little script-magic, you can easily make it download a list of videos or all the videos in a channel.

EDIT: No need for script-magic. There are options allowing you to specify a file with video-URLs. And if given the URL of a playlist or a channel, it'll download all the videos there. Despite the name, `youtube-dl` works on numerous video-sites.
There are a plethora of Firefox and Chrome add-ons that you can use to download flv (Flash Video) from the web, including youtube. I would start there.