If you're a fan of horde mode/twin-stick shooter games, you may want to check out the recently released A Mass of Dead. Sure, it uses zombies for enemies, but really, the kind of enemy you're "shmuping" doesn't matter; just how you go about doing it.
Supporting up to four player online, players will fight their way through 30 stages and/or three challenge stages. There are boss enemies to fight, special zombies that drop unique equipment, and a general store where you can purchase new guns.
The title is available for PC, Mac, and Linux users via Steam for $9.99, and it supports online play across all versions. So if you're all "hi, I'm a PC" and your friend is "down with the man! Viva Linux!", you can still enjoy shooting zombies together.