Apparently, YouTube has been quietly experimenting 3D videos, and some people have been posting all kinds of 3D videos which you can view either by using good old paper cut-out "3D glasses" or simply by crossing your eyes.
For me, the crude "cross-eyed" mode works the easiest. If you wear prescription glasses, take them off, and simply put your face close to your computer screen, and just stare at one of the pictures, until two images merge to create a 3D effect, such as the following video:
This one works with my ColorCode 3D paper glasses (blue-amber method), which I cut out from some magazine advertising certain 3D TV show:
This guy, BlackSharkfr, has posted quite a few 3D videos on his channel.
Here is a blog post that discusses this new YouTube fun feature over at Google Operating System.
Basically, the youtube tag, "yt3d:enable=true", will enable the 3D player, and by putting "yt3d:enable=true" in the YouTube search box, you get a bunch of 3D videos, like here.
UPDATE 7/22/09: Now YouTube has made it official and talked about how to make 3D videos in a blog post. The method of making 3D videos is described by the YouTube employee who took this up as a personal project in an "easier said than done" way, as in, before rushing out to make your own 3D videos, you still need to poke around and experiment a lot. From his post:
Some basics around shooting 3D videos (this isn't easy, so patience is key):
* Use two cameras arranged like a pair of eyes.
* Start both cameras recording simultaneously.
* In your video editing program, place the footage for the left and right eyes together in the frame side by side, with the right eye on the left and the left eye on the right.
* Upload your video! Edit your videos tags and add yt3d:enable=true. If video is widescreen, add yt3d:aspect=16:9 too.
Instructions from YouTube on how to view 3D videos:
First, you need to select which mode to view them on this page. Or, on the video page, look at the bottom of the display pane and click on the "3D" button to select your mode, as in, what kind of glasses you have around. For instance, I have both Red/Cyan and Blue/Yellow paper-glasses, which were cut from magazine ads.
And how to upload 3D videos to YouTube: YouTube help page on uploading 3D videos.