So this mod for the military-themed shooter Arma II has been making the rounds on video game boards and racking up quite the following. It's single-handedly propelled Arma up to #5 on Steam's top sellers list. Servers are apparently at capacity and the game is still in the early alpha stages and not without bugs but complete enough to be tons of fun.
So what's the big deal? Play alone or band together against the zombie hordes! Help others or fuck everyone over! Scavenge for scarce supplies and vehicle parts! Play in a realtime 24-hour day/night cycle! Use stealth to avoid zombies! Awesome sound design to keep you on the edge of your feet! Casual players need not apply as once your character bites the dust, you have to start over from scratch again. And you can bleed out slowly from injuries and break bones, hindering you and ratcheting up the suspense. It's for the hardcore, yo.
The multipart in-game The Days Ahead series of videos illustrates the emergent gameplay pretty well. Start with Part I
There's also this popular short vid where two bandit players rob a noob for his supplies at gunpoint.
And there's more info than you'll ever need about it at Something Awful's forums.