Portal is free till the 24th on Steam! I think it's publicity for Portal 2 AND for Steam now being on Mac and all the games working on both systems. So, having only played the 360 version, I'm totally stoked! That's pretty boss.

http://store.steampowered.com/freeportal/

Edit: Yeah, they confirmed it's yours to keep forever, just have to download it before the 24th. Sweet deal.