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.
Hopefully this should make things considerably more obvious to new players lol!
The one gun nobody knows how to use at the start.