Single Player - System Shock 2 (nothing can beat its atmosphere), Half-Life 2
Multiplayer - Quake 3 Arena, Alien vs. Predator
We had a nice little LAN session which had lost its drive around midnight, until someone suggested we should install Alien vs. Predator for a short multiplayer match. Seven marines hunting down one Predator... or so was the plan. Due to the level design, the lightning (or lack thereof) and the speed of the Predator, we were more busy shouting "Contaaaaaaaact!" and gunning each other down instead of the Predator. That was a hell lot of fun! 😎