If you go to this video, you’ll notice a drop-down that includes some options for red/cyan and amber/blue 3D glasses and some options that don’t require glasses

Wikipedia explains “Stereoscopy, stereoscopic imaging or 3-D (three-dimensional) imaging is any technique capable of recording three-dimensional visual information or creating the illusion of depth in an image. The illusion of depth in a photograph, movie, or other two-dimensional image is created by presenting a slightly different image to each eye,”

To enable the 3D player, you need to add the following tag to one of your videos: yt3d:enable=true. Pete, a Google employee, has more information about this experimental feature:

I’m the developer working on the stereoscopic player as a 20% project. It’s currently very early, hence the silly bugs like swapping the eyes for the anaglyph modes. A fix for this is in the works.

The current tags are provisional and may change or expand. They are:

yt3d:enable=true Enables the view mode.
yt3d:aspect=3:4 Sets the aspect of the encoded video.
yt3d:swap=true Swaps the left and right sources. You may need to add this to videos when the player with fixed anaglyph modes ships