Duke Nukem 3D's sister game. It shares many similarities to Duke3D, though it plays completely different. Rather than carefully balancing ammo like in Duke, this game tries the opposite and makes health extremely scarce, but gives you a ton of super powerful weapons with huge amounts of ammo. To put it in perspective my most used gun in this game is the rocket launcher rather than the shotgun like most old games. Be sure to save often if you play this as being killed with 100 health is not terribly uncommon. The game in general is significantly more difficult than Duke as well (except Duke episode 4).
MP is also pretty entertaining, or at least was back in the day. Good luck getting a DM together now.