Polarized 3D glasses create the illusion of three-dimensional images by restricting the light that reaches each eye, an example of stereoscopy which exploits the polarization of light.
To present a stereoscopic motion picture, two images are projected superimposed onto the same screen through different polarizing filters. The viewer wears low-cost eyeglasses which also contain a pair of different polarizing filters. As each filter passes only that light which is similarly polarized and blocks the light polarized in the opposite direction, each eye sees a different image. This is used to produce a three-dimensional effect by projecting the same scene into both eyes, but depicted from slightly different perspectives. Since no head tracking is involved, several people can view the stereoscopic images at the same time. (source wikipedia)
Polarized glasses are pretty cheap
Altough the glasses are cheap, the production and other hardware might be more expensive