[flohofwoe]

The classic Alien vs Predator (I think?) had me literally falling out of the chair when a facehugger jumped at me :)

[jasonwatkinspdx]

During the LAN party era my friends and I played the crap out of that series. The engine wasn't as sophisticated as what iD et all were doing, but worked perfectly fine for the gameplay. We'd spend hours trying to one up each other in deathmatch.

Another gem of a game I think a lot of people overlooked that has some similarities is Natural Selection 2. This is a sort of aliens vs space marines FPS and RTS hybrid. It never really blew up but still has a community.

[_carbyau_]

My friends and I only bot bashed. NS2 was much desired but fell flat for us.

[cptnapalm]

Only game I ever played where I remained unmoving in one corner for half an hour and was not bored.

[wmanley]

Something I loved about that game was that stopping didn't make you safe - if you stopped for too long the aliens would come and find you. This lead to a intense experience where you had to keep going - even when terrified - because stopping was even scarier.