Machinarium

Diminutive robot protagonist Josef finds himself in what appears to be a junkyard. After a brief tutorial in which actions and abilities are demonstrated, it is made clear through a few cartoonish flashbacks that some bad robots wearing black hats kidnapped Josef's girlfriend, and strong-armed the little robot himself out of the robot city. Josef then proceeds to re-enter the robot city in pursuit of his lost girlfriend, only to discover that the Black Cap Brotherhood has planted a bomb in the robot city. Josef must then solve a series of extremely creative visual puzzles in order to free his lady, disarm the bomb, and save the city. The soundtrack is particularly amazing, the artwork is charming, but "Machinarium" is the sort of experience that cannot really be described in any way that will do it proper justice. Just play the game.
Bioshock

Filled with dieselpunk eye candy and tough moral decisions, this eponymous first chapter of the "Bioshock" series is basically a big middle finger in the collective face of Ayn Rand enthusiasts, proving to many doubters outside the gaming subculture that videogames could be more than just mindless gore-fests.
The player assumes the role of Jack, who has come upon the fallen underwater city of Rapture, built by uber-capitalist businessman Andrew Ryan. Due to an increasing discrepancy between the rich and the poor, and human dependence upon a substance known as ADAM, Rapture has suffered a catastrophic revolt. Jack has survived a plane crash over the Atlantic Ocean only to find his way under the surface to Rapture, guided by a man named Atlas. Jack must then make his way through the city, encountering Little Sisters, the carriers of the ADAM, and Big Daddies, the guardians of the Little Sisters. Throughout a series of quests, the truth is slowly revealed to be even stranger than the initial setup.
BioShock Infinite will also soon be hitting the MAC Store in Summer 2013
Left 4 Dead series

Though both of these titles have been out for a few years now, they were only ported to OS X and made available through Steam just last fall, to the enjoyment of all. There are obviously a plethora of zombie-related games out there, and no one would blame a person for being sick and tired of all the lurching and shambling. However, both of the Left 4 Dead games are particularly intriguing in that the co-op mode doesn't just involve playing together, or helping each other out. In certain instances, a player actually needs his or her co-players, like when the player respawns in a locked closet, or needs healing but is not carrying any form of medicine. Teammates can bring one another back from the brink of death with a defibrillator or lure the infected away from other teammates with a pipe bomb. So basically, short of a space virus that actually causes the zombocalypse to happen, this is probably the most realistic simulations available in terms of the need for teamwork. Even though the likelihood of magically finding guns and defibrillators lying around in real life is kinda dubious.
Portal Series

Originally bundled in Valve's 2007 Orange Box for PC, Xbox 360 and PS3, this first person puzzle-platformer was intended to be a small bonus game. However, it ended up being so wildly popular that it was ported to OS X and released in May of 2010 via Steam. In a series of jump, gravity and timing puzzles with teleportation as the main mechanic of gameplay, the player (who we eventually learn is a woman named Chell) must overcome increasingly difficult test chamber scenarios at an Aperture Science research facility. Chell is at first guided and then taunted by an AI named GLaDOS, who promises "cake and grief counseling" should Chell complete the experiment satisfactorily.
Like its predecessor, Portal 2 is also very much worth playing. Set hundreds of years after the events of the first game, the Aperture Science labs are in a dilapidated state, and allow for you to discover what happened at the laboratory once and for all.
The clever gameplay mechanics from the first game return, and are bolstered by a host of all new puzzles involving various forms of liquid paint, lasers, and other physics-bending materials.

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