In case you haven't noticed yet, Valve has made Portal absolutely free for all steam users until May 24. They say it is in celebration of Steam's debut on the Mac, but it is undoubtedly an effort to create more buzz and anticipation for Portal 2. Regardless of their motives, I would definitely recommend downloading this game if you don't already own it, but do it soon! Here's the URL of the promo page: http://store.steampowered.com/freeportal/