Section 8: Prejudice has an amazing base defense mode. You can use a variety of customized loadouts, there's a semi-class-based system (depends on the gear you carry and the way you customize your suit), and you have a variety of deployables to use based on a single-match currency point system. Most people say the game's pretty mediocre (and I suppose it is) but I find it entertaining. I have no idea if you can play coop cross-platform, though (I own it on the PS3).

Oh...it might be Steamworks for PC, I don't know.
I played LFD2 for an entire weekend then haven't picked it up since. It's just mindless violence in an incredibly linear shooter, and the AI director isn't anywhere near as impressive as they make it out to be.

It's actually fairly simple. You have a chokepoint->path choice->chokepoint system and most path choices have two or three options. The AI director picks one of those. When spawning enemies, if you have high health and ammo, it spawns elites, and if not, it just swarms you with regular enemies. That's not really stunning AI - most games don't bother with tweaking spawn rates because they want you to feel a sense of accomplishment if you maintain high health and ammo, rather than smacking you down for it.

You might also consider Alien Swarm if you have group of people to play with, it's ... basically more complex L4D. It's free and does require Steam, but ... well you don't have to give Valve any money for it so that should be fine, Steam really keeps to itself.

I have been hearing a lot of things about Dino D-Day recently...


Jokes aside, I have played a bit of Section 8 Predjuduce on the 360, and agree with Runehamster about the base defense mode being awesome. You might also check out some of the free-2-play shooters out there. I know a few of them have co-op modes against AI enemies in them. The quality of those games isn't as great, but they are free. Im not familiar enough with any of the current ones to point you in the right direction.

Yeah, but I don't see why that would be a problem for multiplayer games. After all you do need to be online to play them and by their very nature they're going to die long before Steam does.

When it comes to multiplayer games Steam is a bonus. That's why everything post-UT2004 uses it.

Huh. I responded to you...but it is not appearing. Inc double post?

Anyway. That is a good game, played it a bunch with some friends. Though it isn't a FPS like L4D so not really sure if the comparison is that good.