Image via WikipediaLifehacker recently gave two tips on the use of YouTube. The first one targets at watching videos from outside the region where they are allowed (mainly US 😀 ). You just need to change the URL and everything works fine.
The other one features a bookmarklet for downloading the mp4 files of the videos. You just need to add it to your bookmarks and on click it will add a download link for the mp4 file.
Furthermore there is a “Better YouTube” Firefox plugin, which already does a lot of the things the articles describe.