First Person

Unturned is a free-to-play cross-platform action zombie survival game with crafting elements.

Strafe is a retro styled pixel art shooter in the Unity engine that purports to offer endless play with procedurally generated levels.

Interstellar Marines bills itself as an old school tactial FPS, and just recently added a solo and coop player campaign mode called the Neurogen Incident.

This procedurally-generated voxel open world game looks like Minecraft on shaders and steriods. It's being kickstarted later this spring, with an alpha expected this summer, and a release later this year for Windows and Mac OS X.It still seems sort of strange to me that developers look at Minecraft and think the way forward is better graphics, but hopefully there's also a good understanding of what makes Minecraft fun.

The successor to the Quake engine-based Nexuiz, Xonotic is a fast-paced multiplayer shooter, similar to Unreal Tournament or Quake 3 Arena. Features FFA and team play.

This squad-based first person shooter is a full release based on a Source engine mod; it runs on OSX and Windows and supports crossplatform multiplayer as well as competitive and cooperative maps.

Built from the ground up for cooperative play and supporting OSX, Linux, and Windows, Left 4 Dead 2 definitely belongs on this list. And for a short time during the holidays in 2013, Steam offers the game for free!

Released on Early Access in December 2013, this first person action adventure RPG  features cross-platform multiplayer on Windows, OSX and Linux. The game looks to be pretty early in development, but seems to be snaring enough people to invest the hours in playing it.

A re-creation of the original Half-Life in the 2007 version of the Source engine, with upgraded high definition audio and video-- up until the Xen chapter, at least. There's no native OSX version, but if you install Steam into a WINE wrapper, you can use it to install the 2007 Source SDK and play it on OSX.

This classic sci fi shooter morphed into the multiplayer powerhouse Unreal Tournament, and supported Windows and Macs back in the day, but the processor and operating system transitions left the official Mac versions behind.
There are two ways to play on modern versions of OSX: One is a Cider version available at the Unrealism Zone; the other is a WINE wrapper available from The Porting Team.
Either allows the Windows game from GOG or Steam to be played on OSX, and cooperative mode is supported. 

The precursor to the Bioshock franchise and the sequel to the classic original System Shock, Irrational's best game ever features the only crazy AI who could give Marathon's Durandal a run for his money: Shodan. 
UPDATE: GOG has System Shock 2 on sale right now for $2.

Markus "Notch" Persson created this block-based game in 2009 using Java. Years later, versions exist for PC, OSX, iOS, Android, and Xbox, with a PlayStation version in the works. 

Durandal and Tycho continue to struggle with the Security Officer and the Pfhor as their pawns, but the real threat is from the chaos embodied in the W'rkncacnter released by the sun-destroying Trih Xeem weapon unleashed by the Pfhor.

Megalomaniacal AI Durandal has captured a Pfhor scout ship and taken it to the homeworld of the enslaved S'pht in hopes of finding the secret of the lost clan that can end their slavery.

The colony ship Marathon comes under attack by alien slavers known as the Pfhor. The only hope for the colony lies with the mysterious Security Officer and three on-board AIs.