Zombie Grinder is the debut game of TwinDrills, an indie game development duo made up of Tim 'Infini' Leonard and Jordan 'Jordizzle' Chewning. It's a multi-platform (windows, mac, Linux!) (also Free!) cooperative multiplayer arcade game, reminiscent in style and gameplay of retro games like 'Zombies Ate My Neighbors' and 'Super Smash TV!', but with the added benefit of some newer game mechanics - achievements, ranking, player-customization, rpg style stats and so forth. The game sports a variety of different game modes, from co-operative 'campaign' style levels, to wave maps and even the classic 'pvp' style game modes - deathmatch, capture the 'bag' and so forth. All of these mods are bristling with a variety of weapons and items, all lovingly displayed in Jordan's classic quirky pixel-art style.
I've got to get this a 10/10, it's really that fun.
There is a lot to do in this game, you can customize your character, participate in different kinds of zombie slaying modes, even PvP.
It also has a great retro style for classic game lovers like myself. Not to mention that this game is only in alpha, so I would expect there will be much more to come.