Portal is game design milestone. Originally only available bundled in with larger cousins Half-Life 2 and Team Fortress 2 in The Orange Box, its creators, Valve Software, changed what first person gameplay could be. It's been a stand alone product for much of the four years since its release, but never an expensive one, usually available for $5 on Steam (also created by Valve).
If that barrier to entry was too high for you, or you just never got around to trying it, there are no more excuses. As part of their educational Learn With Portals initiative, Valve has made Portal completely free to download on Steam until September 20th. The goal of the program is to teach kids about math and science through the lens of designing Portal levels, and letting as many kids have access to the game as possible will further that goal. As Valve themselves said of the intitative:
"Using interactive tools like the Portal series to draw them in 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!"
Portal is four years old, and will run on any decent computer from the last five, so technical constraints are probably not an issue for you. It even works on Macs! There's not much I can say about Portal that hasn't been said ad nauseum. Only this: if you play games at all, haven't played it, and don't now, you are willfully ignoring something really special, and that is tragic.