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Killing Floor yo
Also Nation Red

Hmmm Terraria

Killing floor is VERY good for stuff like this. However its level based, not mission

Alien Swarm.

Maybe you should just wait for Firefall. Or get into the BETA if you can.

I'll try to think of an alternative though.

Ah, Borderlands, and 2 when it comes out.
Orcs Must Die 2 will also have co-op, but that's only 2 people I think.

Borderlands.

serious sam 3

minecraft is all of that at the same time :P

A Good Co-Op ? i'd say Nation Red, its a nice Kicking ass game (real cheap too!) To pass the time with your friends!

Left 4 dead 1?

Dungeon Defenders

Garrys Mod, you can make your own war, your own Zombie infestation, your own mission!

Also Cry Of Fear, a HL1 Mod!

From what I've understood, you're looking for co-op games. Here's a list of my favourites.

Killing floor is great, also, Dead Island is a great game, in my opinion. I've no idea why so many people dislike it, but I guess it's either a love or hate relationship with Dead Island. It's GREAT if you have at least one friend to play with you.
Borderlands is another great coop game, easily 40 hours on that. Saints Row: The Third is another fun co-op game, but it is quite childish and immature, don't know if you're into that.

Good luck and have fun. :)

portal 2 coop is very fun with friends :)
But if you really want pve ill i'd reccomend borderlands or killing floor

Go for dead island!

Magicka is quite entertaining

Borderlands would be my first choice but as that has been said I am also a fan of Darkspore. =)

Get Syndicate

Magicka Collection

Although it doesn't really fit the style you're looking for I highly recommend Mount & Blade Warband. The singleplayer has personally lasted me 30+ hours since I bought it a few weeks ago. It also has multiplayer so there's really no reason not to pick it up.
I tend to recommend this game a lot since I haven't found anybody that actually dislikes it. One friend I told about it ended up getting over 100 hours out of it and he's still going, not to mention that the game is highly moddable.

Titan Quest Gold. I'm dying for Darkspore!

Borderlands

Killing Floor definitely.

Orion: Dino Beatdown is awesome and it's only $10. I'd also recommend that you get Borderlands, better if you get the GOTY Version. After like 2 years since I first completed it, it still remains one of my favourite games. Dead Island is also pretty good. I have all 3.