Only a terrible person will not have played Portal by now, and you're surely not a terrible person, but should you happen to know any dreadful people, you can help them remedy that by pointing out that Valve's splendid first-person puzzler is free on Steam for the next few days.
Portal's been made free as part of Valve's Learn With Portals program, which encourages educators to use the game as a learning tool and to spark interest in students. According to Valve, using interactive tools "makes physics, math, logic, spatial reasoning, probability, and problem-solving interesting, cool, and fun which gets us one step closer to our goal--engaged, thoughtful kids!"
The offer of free Portal only runs until September 20, so inform those beastly louts sooner rather than later. Or, you know, children who need a swift boot up the jacksy at school.
See, here's a group of schoolchildren visiting Valve to play Portal, during which time they all stopped torching cars, selling drugs, and getting pregnant, thanks to feeling inspired:
As noted below, all of the Steam for Mac launch titles offer a free Mac version to those that already own them on Steam PC thanks to the "Steam Play" feature, "which allows customers to purchase a game once and play it on all Steam supported platforms."