Well, I was pondering what it would take to do a 3D projector. It is pretty simple if you take a "side by side" image format. You can do it all in optics.
You would need a simple lens to double the width and a pair of mirrors to overlay one half on to the other. You would have to use a non polarising technology like DLP or some such (not LCD) and add polarising filters to each side. Then simple polarised (cinema style) glasses will work without all the usual flicker you get.
But doing a search it looks horribly like projectors are going down the "shutter glasses" route. Why? It just makes more expensive active glasses that are less comfortable to use and gives horrid flicker effects. Projects *can* use polarisation which LCD TVs cannot easily do so why are they not doing this. Oh well.