Anaglyphs are 3d images which must be viewed with colored glasses. The principle can be explained easily: The two half images, one for the right and one for the left eye, are colored in complementary colors, for example red and green (or red/cyan, red/green,...) and added together. The red/green colored filters in front of your eyes only transmit one of the colors in the image while absorbing the other one. Hence, the green color on the image appears as black on the eye behind the red glass while being indistinguishable from white on the eye behind the green glass. More...

This trick is really simple: Just put the two images for the right and the left eye next to each other and look at them in a way that one eye only "sees" one image.

There are two methods:

The parallel view: Image size is limited to 6cm width because the eye distance is about 6.5cm. Instead of looking "at" the images, you look "through" them with your eyes being parallel. Hence, the right eye sees the right image and the left eye the left image. It takes some training to do that instantly. More...

The cross-eyed method: It works like the mehod above but with the right/left images swapped. Hence, you need to look at them cross-eyed (right eye sees image on the left). This allowes for larger image sizes when viewed from a larger distance. More...

The image for the left and right eyes are projected on a plane through two orhtogonal linearly polarizing filters (i.e. the right image is horizontally polarized while the left one has vertical polarisation). In order to see the 3d effect, you use glasses with (linear) polarisation filters. More...

The eye has two sorts of light receptors, the rods which work in dim light and the cones, used for bright light. In 1922, the German astronomer Carl Pulfrich discovered that the perception process of the rods takes fractions of a second longer compared to the cones. Using so-called Pulfrich glasses (which have simply one clear and one dark glass), a 3d-effect can be achieved as long as the scene is moving because the brain is combining the instant (bright) and the delayed (dim) image.

For that technique, the images for the right and the left eye are displayed in quick alternating sequence. The viewer has special shutter glasses which "close" the right or left eye at the correct time. If the frequency is high enough one will not see the flickering.

I'll explain this when I find the time to do so :)

Special more expensive techniques include 3d-LCD-monitors: These can be viewed without special equipment as long as you stay in front of your screen; if you move to the side, the 3d effect vanishes until it finally inverses. Special improved versions track the location of the viewer in space and update the screen to keep the 3d effect. Other versions are capable of handling more than one person simultaniously.

3d helmets. Versions with one screen for each eye (and some lense system, of course) and versions which use lasers to directly project the image onto the retina have been invented.