Or...Signa said:Descent just released on Steam. You shouldn't need to look further than that unless you get motion sick.
Edit: and yes, as you just said, Doom is always a good option. It's amazing what happens to a FPS when you remove the need to aim and reload. It sounds really bad when I put it that way, but when all you need to do is face someone and press the fire button, the game becomes a frenzy instead of some sort of casual-friendly mess. The "death maze" part is just icing on the cake, because exploration is always fun.
http://www.gog.com/game/descent_1_descent_2
http://www.gog.com/game/descent_3_expansion
Also, I disagree heavily with the implication ruling in this forum that shooters that aren't Doom or Build Engine are more 'casual' than the aforementioned. Maybe we like our shooters different. Maybe we like both types.