Shattered Horizon is a multiplayer team FPS set in space. No, really. Not inside a space station or on a spaceship, but actually in space; as in, you're in a pressure suit shooting at other guys in pressure suits. The most important thing to know is that you have six degrees of freedom: forward, back, side-to-side strafing, and vertical strafing. Inertia is also modeled: if you take your hand off the keyboard, you continue moving along your last vector at a constant velocity, unless you hit something and rebound off. Movement takes a few minutes of getting used to, but quickly becomes second nature. You can toggle "silent running," where you don't show up on enemy radar, but you also don't have any radar yourself. Handy for drifting through open space plinking enemies from a distance, less so for combat in tight quarters. Thankfully the developers, Futuremark Studios, thought to restrict players' ability to drift way outside the combat zone and snipe from an unreachable distance: drift too far from whatever object forms the arena and you start taking damage from micrometeoroids, which will kill you after twenty seconds or so.
The maps are well done and a lot of fun. Each one I've played has a mix of large open areas and small, cluttered areas, and feels very realistic. Weapon selection is a little anemic compared to more over-the-top shooters, but very much in keeping with Shattered Horizon's hard sci-fi milieu: every player has a pick (melee weapon), pistol (backup sidearm), and grenade pistol (shoots six different types of grenades; more useful for area denial than actually damaging anyone). You also get to pick one of five primary weapons: a railgun (obligatory high-damage, low-rate-of-fire sniping weapon), machine gun, assault rifle, submachine gun, or shotgun. The railgun and machine gun are "non-suit-integrated," meaning they're more accurate when you're attached to a surface; the others are suit-integrated, and more accurate when fired in flight.
I'm having a lot of fun with this game, enough that I ponied up the $9.99 it's going for on Steam this weekend. You can download and play for free all weekend, and if you're looking for something very different and a little more tactical than your average run-and-gun shooter, I strongly recommend Shattered Horizon.