YouTube culture is fertile ground that implicitly encourages video replies and mashups. To do that, people need a way to download videos. These unofficial Google Chrome extensions (not in the Web Store) makes it easier to download YouTube videos in formats like MP4, FLV, etc:
Download YouTube Videos (by Joshsmith9) v1.0.01
Related extension in Chrome Web Store:
Video Downloader professional (from http://www.startpage24.com) version 1.97.37
Caution: The download of YouTube videos to hard drive is locked because of restrictions of the Chrome Store. The download of protected videos (RTMP protocol) is currently not possible.
[update 11/22/13] Download and install Torch Browser (Windows/Mac) - built on the Chromium technology platform, Torch Browser has convenient built-in media downloading! Download YouTube videos with the greatest of ease!