Who says you need magic mushrooms to have a strange game of Doom? If you're tired enough doom can seem very strange indeed. I remember at a LAN party where I was playing with some other guy in Doom. I hadn't had sleep for a loong time but we played regardless. It was really apparent I was in serious need of sleep. I could see suddently something would be coming at me and I'd just be shooting with whatever weapon I was holding. When running around the mazes I couldn't find my way, I remember running around in circles without being able to find my way out of the room.
If you talk about more than the scarey effect you could talk about shock effect. Some games really do manage to shock you quite a bit. Here again I'd like to pull out Alone in the dark (1). If you had an old SB soundcard and the disk version of the game then you could always hear when something was attacking you. You'd be walking around wearily almost dead hoping to avoid confrontation, and then suddently you'd hear that sound, I think the closest to it would be a thing which I only know as an orchestral blast whereafter the music would start playing. This did give me quite a shock or 3 or 5 while playing. The CD version on the other hand had all music on CD and the effect there is gone
Another game with a shock effect (at least for me, I'm easily shocked) is Half-life. I remember more than one time when I got shocked as a headcrab suddently would jump at me, or those things in the ceiling would snag me. And many of the effects around you. If any of you ever played the game that far you'll know about some very large thing, first time you encounter it is at the rail split where you see it finishing off a couple of soldiers with ease. You end up killing it by running down a hallway with your life as stake while it thunder down the hallway after you, just hope you turn on the power soon enough so it'll get caught and die. And those thingies on rods, the first time you encounter those. You walk into a room and you notice a scientist looking at the window screaming "no no no". Look at the window just in time to see something large bust through the window.
There are many other games with good shock effects, and I found 3D games actually have a plus there because things can sneak up on you without you knowing it before you get attacked.