Portal out FREE for Macs and PCs
That's right! Valve is offering Portal for free on steam currently to advertise the new compatibility with Macs. The link to download is http://store.steampowered.com/freeportal/
Coming from someone who's played Portal, I was glad I BOUGHT the game. Since you have the chance to get it for free, if you haven't played it yet, I suggest you do so!
I want to thank Pretendica for pointing out this should be posted in the News Section