YouTube introduced a stereoscopic player for watching videos in 3D. If you play a 3D video a drop-down menu will show options for red/cyan and amber/blue 3D glasses and some options that don't require glasses. Here's a brief overview video.
To enable the 3D player, you need to add the following tag to one of your videos: yt3d:enable=true. Peter Bradshaw, 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.
You can try the new feature by searching for yt3d:enable=true.