Go get it now if you don't own it! This isn't a temporary deal. It is a temporary offer, but once you buy it (for free) you own it. Even after the due date (May 24th) is passed. Go and get it now even if you're computer can't play it! Someday maybe you'll get a good enough computer so you can play it and you'll thank your lucky stars that you managed to take advantage of this incredible deal while it was free!
Those of you who don't know what Portal is....you're beyond help. Just give it a try.