I happened to purchase this game called Contagion (
Steam Store Link) when it was on sale for like $5 and I have to say...it's damn good!
It supports up to eight players, is focused on survival and completing objectives such as escorting survivors, escaping...something (depends on the map) and it also has a PvP mode. It's not all about running and gunning - ammo is sparse...very sparse. And you have to make your shots count as headshots are the only way to effectively take down a zombie (duh). Also, when you die as a survivor you respawn as a zombie and your new goal is to take out the survivors. As a zombie you can recruit other zombies and create your own horde. And if you die as a zombie....you come back as a cockroach.
Supports community content and wait for it - SUPPORTS LAN PLAY! That's right....offline mode LAN servers.
It is currently $19.99 but I think it's worth every penny.