It was just last week we saw a hypothetical video about this. Last week! Today, we have something very similar, only this time it's something you can actually play.
Doom Reborn is a giant mod that basically brings the original Doom (and Doom II) to the Doom 3 engine. So as you can see, you get the same level design and same blocky vision of a hellish Mars, only now with a fuller range of movement and some more three-dimensional bad guys.
Keep in mind this is a "Pre-Beta" build, so things could get a little rough (some of the bad guys in the clip below, for example, don't fire back).
You can download it right here.