Link.
It's in Alpha, and you can't actually buy it. However, you can buy one of their other games, which they admit aren't awesome, and in return you get a lifetime license to Project Zomboid, which gets you access to begin playing the Alpha right away.
I hear the Alpha is pretty entertaining and the promise of the game genuinely makes it a contender for best zombie game ever...
I'm thinking of joining up this weekend. Thoughts Monday if I do.