Left 4 Armies
Honestly, I'm a rather surprised this didn't happen sooner. The cast of Left 4 Dead have invaded the Zombie Army Trilogy. All eight of them. Because they just couldn't give it a rest with the zonbie ass kicking action I guess. Which isn't all that surprising, Bill does have a kickass beard. Best of all, the whole lot of them are being added as a free update for all Zombie Army Trilogy owners on Steam. Including those who buy it during the current 66% off sale.
Just imagine if they added in some characters from Telltale's The Walking Dead too. With Bill and Kenny sporting all the powers of the beard, their efficiency would be magnified and thousand times, and you could shoot zombie Hitler in the dick without even needing a gun!
There's even a free comic tying the two, because why the hell not?