I enjoyed Space Siege, even though alot of people gave it a bad rap.

You might also enjoy the Alien Breed series available on Steam.

Or the Alien Shooter series

What expansion? As far as I know there is just a sequel, or am i missing something?

Survivor is a "Semi-sequel" but more like a stand alone expansion. Sort of like Age of Wonders Shadow Magic is not called "Age of Wonders 3".

Anyway, I'd recommend Alien Shooter series myself - though they're more about humongous mobs of enemies and less about the atmosphere. I guess you could say that Alien Shooter = Doom (especially Doom 3), while Shadowgrounds = Serious Sam.

Another game that is kind of similar is Alien Swarn which is free on Steam. It's primarily for co-op play though so it doesn't have a very strong story. But it's free, so there's no harm in trying it out.

Well if you like 3rd person shooters, you might try a classic, Crusader. Which is truthfully much more fun then this game but a lot older. Don't let that throw you, no shooter will ever beat that game for fun once you have a rocket launcher. If you drop your weapon and pick it up again you get unlimited ammo. Which is good because by the end all the enemies, even grunts start carrying real weapons.