This one is a good extension for downloading Videos from Youtube. It supports downloading Videos and automatically names them:  ACCOUNT NAME – VIDEO NAME . flv/mp4.

You can download it from here

http://www.chromeextensions.org/music-videos-photos/one-click-download-links-youtube-only/

Here are some info about it from the author:

One-Click Download Links – A Google Chrome Extension for downloading YouTube videos easily!

Version 1.24, launched August 10, 2010

CURRENTLY SUPPORTS YOUTUBE ONLY, more popular video sites coming soon.

Now go to any YT video page. You’ll see a list of links to download vids at the top of every page. Just click the link like normal, and it will save the file for you, properly named, instantly!

Files are automatically named like this: ACCOUNT NAME – VIDEO NAME . flv/mp4

This extension does not have “AUTO-UPDATE” enabled yet, so please check back every week or so for updates.