An interesting side-effect of YouTube's recent push for higher quality videos is that most videos can be downloaded as MP4 files directly from YouTube. Until now, you could only get FLV files from your browser's cache or using one of the many websites that let you. download YouTube videos In fact, to download the MP4 files, you need to use the same URL like for FLV files and append "&fmt=18":
Watch: http://www.youtube.com/watch?v=MvkQm7Dn3E0
ID is the video's identification value, SIGNATURE is a value that prevents you from downloading the file just by knowing the ID. You could create this URL by looking at the source code of a YouTube page, but it's much easier to automatically generate it.
Add a bookmarklet
One way to download videos is to save the following link as a bookmarklet by dragging it to your Links bar (in Firefox, Safari) or right-clicking and adding it to your favorites (in Internet Explorer, Opera):
Watch the video below for more detailed instructions:
HTML-video comes later
http://www.youtube.com/watch?v=MvkQm7Dn3E0
When you want to download a YouTube video, just click on "Get YouTube video" and the bookmarklet adds a small link below the video's description: "download standard MP4". Click on that link and you'll download the video. For some videos, you'll also see the option to download the HD version.
The bookmarklet only generates a download link and it no longer starts the download automatically.
By the default, all the videos are saved as "video.mp4", "video(2).mp4" or similar names. If you want to use a different name, right-click on "standard MP4" and select "save target as" or "save link as", depending on your browser.
Install a Greasemonkey script
Clicking on the bookmarklet to get the download link may be annoying, so this Greasemonkey script is a better alternative because it adds a download link automatically.
To add the script, you need to install the Greasemonkey extension for Firefox or a similar plug-in for userscripts. Opera has built-in support for userscripts, so you only need to go to Tools > Preferences > Advanced > Content > JavaScript options, select the directory where you will put your script and copy the script to that directory.
YouTube's MP4 files have a higher resolution, stereo sound and can be played with applications like VLC, MPlayer, itunes, QuickTime. Not all YouTube videos can be downloaded as MP4 files and the fallback format is FLV.
Note that YouTube's terms of use require you "not to access User Submissions or YouTube Content through any technology or means other than the video playback pages of the Website itself, the YouTube Embeddable Player, or other explicitly authorized means YouTube may designate," so these scripts could break YouTube's policies. The scripts only download files that are used by YouTube's player, so you may also find the videos in your browser's cache.
Update: You no longer have to right-click and select "save link as" because YouTube sends the right MIME type. Just click on the download link.
Update 2: If the video is available in HD, the script shows the download link for the HD version. Thanks, pepemosca!
Update 3 (Oct. 31, 2009):
I updated both the script and the bookmarklet. There are two changes:
1. If the HD version of a video is available, you'll see a separate link for that version.
2. The bookmarklet no longer starts the download automatically. It only adds the download links. This way, you can download either the standard version or the HD version and you can also pick a name for the file (right-click, select "save target as..." or "save link as...").
Update 4 (Dec. 2009):
* added support for 1080p HD videos.
* no longer showing download links for VEVO videos, which use protected streaming (RTMPE) and can't be downloaded.