So this may sound a little geeky, but I'm kind of excited for Nintendo's 3DS. I've got a DS Lite, and it's worked great for me, so I didn't bother with getting a DSi, but the 3DS is something completely different.
Initially, I couldn't even imagine it was possible to have a 3D image on a 2D screen without using glasses. In order to see in 3D, each eye must receive a slightly different image. When you are looking around you, objects are actually present in three dimensions, and so each eye can see a slightly different angle. In order to make a flat image seem 3D, you need to have two images in one, and specifically show one to each eye. Normally, this is done by using glasses.
In the early days, the glasses had one red and one blue (or green) lens.
Everything, including red-colored things, looked red through the red filter, so the eye behind it only saw the image in blue. The opposite was true for the blue filter.
Pic from 3dglassesonline, linked above.
Next came glasses that appeared almost clear, but with a slight grey tint, like sunglasses. This technology was used mainly in movies. These lenses were polarized perpendicularly with respect to each other. Both images were projected onto the screen, but one with horizontally polarized light and one with vertically polarized light. If you tilted your head to the side, you would see both images with both eyes.
Pic also from 3dglassesonline.
That problem has been fixed by the current set of glasses, which use circularly polarized light (one lens clockwise, the other counter-clockwise). The projector switches between the images for the left and right eyes, and a filter in front of it switches polarity at the same time. The theater then requires only one projector. This is what is used in most movie theaters today.
The 3DS doesn't require any glasses at all. While Nintendo has been keeping mum about exactly how it works, chances are it uses the parallax barrier LCD developed by Sharp, which has "windows" at certain intervals that let light through. The distance between the windows is set so that each eye sees a slightly different image. Granted, the viewing angle is small, and this wouldn't work for movies, but it's still pretty cool, and it opens up doors for new ideas and games for the DS. Nintendo is pretty damn innovative, eh?