Back in 2005 there was a mod for Unreal Tournament 2004 called Killing Floor. It's a zombie survival scenario where you and five other people group up and defend from wave after wave of oncoming zombies. For each person after the first there were 20 more zombies added to the wave. This went up to six people. After each wave was completed there would be more and more zombies. It wasn't too terribly great, but it passed the time as a zombie mod.
Fast forward a few years. It's 2011. Some where back in 2009 or 2008 they released the game on Steam in pretty much the same format. Things are cleaned up slightly, graphics are retooled and prettier, and the guns handle and actually aim a whole hell of a lot better than when they were part of the mod.
But what gets me addicted to any game is leveling something up. It could be the world's most atrocious game with virtually no content. But if you tell me I'm making progress through a leveling system of some sort, I'll suffer through until I feel I've reached what I want to reach or unlocked some mythical ability that isn't dog shit compared to the first skill you gain. But in this you choose your class, something along the lines of a Team Fortress 2 setup, and then as you progress the guns you buy cost less, do more damage, and reload faster.
Anyone else play Killing Floor? Or any other Steam games? I'd gladly do without Steam, but seeing as most any game I buy off the shelf now requires it, it's kind of a pain in the ass to avoid.