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