Yesterday I read a post on mashable.com about YouTube's new 3D video feature and I read about all the new 3D technologies that were in CES 2010, so I decided I should try implementing something like this on XNA.
Of course the easiest way to display 3D images is using the cross-eyed stereoscopic approach where the output is 2 images displayed side by side and you cross your eyes so that your right eye sees the left image and your left eye sees the right image, so you see a 3D image floating between your eyes and your screen as a result of these 2 images combined and interpreted by your brain.

To be able to do this in XNA, I used 2 cameras for my scene instead of one, both looking at the same point but their positions are slightly different, there is an angle of 0.08 radians between the 2 cameras, then I rendered the scene twice (once for each camera) and took the output of each and displayed them side by side, and the result was Great !
The only problem is that it's not easy for everybody to cross their eyes and it's quite tiring for the eyes, so I decided to implement another way for displaying 3D images, I chose the Red-Cyan Anaglyph approach were the output is an image with some parts tinted in Red or Cyan and you wear special 3D Glasses where (usually) the left eye is covered with a red filter and the right eye covered with a cyan filter, and when you look at the image with these color filtering glasses you see a 3D image parts of it appear to pop out of the screen and others appear to be behind the screen.
To implement this I made some research to know how exactly the 2 images are filtered and how they are blended together, I kept reading stuff from Wikipedia and other places then try to impement them on XNA until it was 5am when I was finally able to get the required output, I put on the 3D glasses (actually I used 2 light filters infront of my eyes cause I don't have the glasses, but it's the same thing) and I saw a satisfying 3D result !
The only problem is, sometimes the colors of the 3D result of a Red-Cyan Anaglyph looks messed up, that's because your left eye sees only the Red Component of the light coming from your screen and the right eye sees only the Green and Blue Components, and your brain tries hard to combine these components to get the color, but for some colors it's just too hard for your brain to interpret from their components, so the result are far from perfect.

Why didn't I implement other better ways, like the one used in the 3D version of the movie "Avatar", simply because they require special hardware, for example "Avatar" in 3D was made with a polarization approach were the 2 images are displayed from 2 different cinema projectors, one emits horizontally polarized light and the other vertically polarized light, and the 3D glasses you wear are made in such a way that each side filters polarization and permits the other so your eyes see 2 different images without any color filtering.
Of course this approach is impossible to implement on the PC since I have no control over the polarization of the light coming from my computer screen, I even discovered that my laptop screen already emits polarized light, I put on the glasses I got when I watched "Avatar" and I saw the laptop screen with only one eye, the other just sees black !
The only way to get very nice 3D images from your computer is to use NVIDIA's new technology it showed off at CES 2010, but it requires a special Graphics card and a screen with a very high refresh rate and special (and expensive) glasses that you connect to the Graphics card, so lets' just hope that it gets popular, cheaper and available to everybody soon

Click on the image in this post to enlarge it, it's a screenshot of the Red-Cyan Anaglyph output, if you have a Red-Cyan 3D glass, put them on and look at the enlarged picture.

You can find below a link to a zip archive containing the XNA project that I made.
NB: you can switch between Red-Cyan mode and Stereoscopic mode by pressing the space-bar.

If you just wanna run the sample without compiling any code, here's a zip archive with only the exe, the 3D model and the XNA dlls, but you'll need dotNet framework 3.5 to run it (I think it comes with Windows 7).

Update:
I re-uploaded both zip files because some people were unable to extract them using Windows Explorer (you had to use 7-zip) and I also added a few comments to the code so now it's more understandable
If anybody has any questions about the code don't hesitate to e-mail me (you can find the e-mail on the About Me Page)