Portal. Everyone's heard of it, probably played it. By now, even the sequel is a bit of old news in the light of ever-increasing new games. If you still haven't experienced Portal, here's the gist: this gives new light to the "first person shooter", probably even to the level that made DOOM redefine the genre. The game, built on the Half-Life 2 engine and sold with it (The Orange Box) is minimalist: you are Chell, a woman in the "Aperture Science Laboratory" testing out a prototype gun that can produce two "portals" (one orange, one blue) that you can enter through instantaneously. Try not to think about the physics too deeply, though. Here's a trailer for it from 2006.
I found this one of the immersive video games in years (of course, I haven't played much A-list PC titles for most of the last decade) and it is very fun. It also redefined the "puzzle" genre for me (along with Braid, where as mostly-static games like Tetris counted as "puzzle" games prior to this).
Despite the minimalist atmosphere (though there is pulse-pounding "shoot or die" environments, no doubt about that), it is well-written, and features good voice acting (a rarity in video games, sadly). It doesn't require an ultra-fast computer to run either, though I recommend that if you have something like a low-end 2010 MacBook, you install Windows on it, as Mac performance is throttled due to a wrapper that translate Windows system calls. It also explores a few concepts in gaming: for example, pathological lying. As in, how can you tell if someone is lying or not? Are they even giving you the right directions?
Portal, due to its age and being hugely popular, is $10 on Steam, $25 bundled with the much larger and substantial Portal 2, and a free demo, that is, if Steam isn't having a sale (you should be able to get it for quite cheap if you just wait a bit). That's one of the few things about it: it requires Steam, which requires you to sign up for it with a distinct username, though there might be a way to get around Steam's requirement, though this isn't exactly legal or supported. Other than that, there's no other DRM.