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
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.