Vanilla DOOM 3, mind you, not the somewhat mangled, more expensive, and distinctly lacking proper flashlights version that was recently released, DOOM 3: BFG Edition. The original, stripped down (and moddable) DOOM 3 was removed from Steam after BFG Edition was released, but after some outcry, id Software told me it was looking into having the game reinstated in some capacity.
Mod support and better-loooking flashlights are reportedly coming to the BFG Edition in a patch, but that one will also cost you quite a bit more. BFG Edition? $29.99. Regular ol’ DOOM 3? Just $9.99.