A sequel to a game that won multiple Game of the Year awards in 2007, Portal 2 pits you against GLaDOS, the artificial-intelligence-gone-bad behind the Aperture Science laboratory that you escaped and destroyed in the original Portal--or so you thought. You once again find yourself to be a rat in an experimental--and malicious--maze. You'll need to solve a myriad of puzzles to avert death and escape GLaDOS, and your only tool is the now-iconic portal gun, an Aperture Labs invention that lets you create, well, a portal that you can use to move objects, enemies, and even yourself between any two places you desire.

This time around, Valve has expanded on the devious and dangerous world of Portal by adding a deeper narrative and an actual cast of characters--besides the homicidal GLaDOS--to help drive the story. There's also a new co-op campaign in which you and a friend, playing as robots named Atlas and P-body, must solve puzzles together or risk becoming scrap metal.

Portal 2 marks the first time since Steam's arrival in May 2010 that Valve has included the Mac in the launch of a major new title; it's also available on PC, PlayStation 3, and Xbox 360. Portal 2 also supports the Steam Play licensing system, so you can purchase it on one desktop platform and play it on the other at no extra cost. If you purchase Portal 2 for PS3, you'll get a Steam code to play it on your Mac and PC (sorry, Xbox owners). You can watch the original Portal 2 teaser trailer below, and the , to get an idea of what GLaDOS has in store for you. You monster.