Though still very much in experimental mode, a YouTube developer has built a stereoscopic player—a fancy way of saying you can already view some YouTube videos in 3D. The project is open for testing now.
In the screenshot above you can see what the video looks like when playing in 3D mode. Embedded below is what the source video looks like when taken with a stereoscopic camera.
To enable 3D viewing, use the "yt3d:enable=true" tag on your video. There are currently a total of 10 3D viewing styles to choose from, including "Focus-Thru", "Cross-eyed" and "Mirror Split", and they work on various videos containing the right tags, like this one.
Other current tags (all subject to change) include:
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. Apologies for the [inconvenience] .
yt3d:left=0_0.1_0.5_0.9 and yt3d:right=0.5_0.1_1_0.9 These tags are very provisional and most useful for fixing up old videos. They set the source area for each eye as pairs of coordinates x1_y1_x2_y2. The scale of these coordinates is 0,0 for the the top left down to 1,1 for the bottom right.
The developer in question, identified as YouTubePete, says that "ideally you shouldn't use left and right", and that for the best quality videos, "make the sources as large as possible within the frame and set yt3d:aspect correctly".
Test out the 3D feature for yourself, and let us know what you think in the comments.