Arma 2's DayZ zombie survival mod is everything your lil' nerd heart desires (probably)

Ooh!  Zambies!

Arma II is a 2009 tactical shooter that most people (myself included) had probably never heard of before a few months ago.  Even fewer people had the desire to play it despite solid mechanics, a huge open world and tons of support for modification of the game -- mostly because it's incredibly realistic and therefore incredibly sloooow.  Also, it had tons of bugs at launch.

Then, all of that changed after word got out that there was a new extensive mod for Arma II which turned it into a zombie survival game.  

The mod, called DayZ, was originally coded and produced by one man (who is now employed by Bohemia Interactive, the company that originally made Arma II).  As of the writing of this post, there are currently 726,882 players who have currently installed the mod and have played it at least once.

Keep in mind, that's around three-quarters of a  million people.  Playing a mod.