Left 4 Dead

There's an inexpensive British Left 4 Dead called Killing Floor on Steam. Quite fun multiplayer game with pretty kick ass graphics. It was me and my squad mates (I was on a server for 20 people) against 10 waves of various zombie mutant creatures, some far deadlier than the others. Each wave was a couple hundred mobs, with the last wave around 500 followed by a patriarch boss.

The maps are sweet and extremely detailed. So far I've fought in a metal foundry, West London, a devastated nightclub, and on a farm.

Every monster you kill gives you cash (lol Brit money), and you can use that cash to gear up at a trader in between waves. Buy better weapons, refill ammo, repair armor, etc. You can also find a few weapons and some armor and ammo scattered about each level as you play.

There are also "classes" you can choose that give you different bonuses with different weapons and/or damage resistances. Pretty generic stuff.

There's a solo, single player mode which sucks. It's not nearly as much fun facing the waves by yourself.

$10 is a jolly good deal for this game. Party on, blokes.
 
What's not to like? You and your group slaughter thousands of monsters while trying not to get split up and surrounded. When the waves are ~700 mutants each it gets pretty hectic.