Everything is rendered in 3D engines these days. The environment is already 3D, it's just ouput to a 2D display.
Any hardware that cares to can intercept the render calls, offset the camera position, render, offset it the other direction, render again, and composite it for a 3D image. This will work with essentially any modern game rendered in a 3D engine without requireing any changes to individual games.
Was looking at a 3D monitor the other day in Fry's that demonstrated a dozen or so modern games (WoW, L4d, Crysis, etc) in 3D display. It kind of sucked as it used polarized lenses, so there was still a ghost image for each filtered lense...but other than that it was pretty impressive.