To celebrate Steam's arrival on Mac, Valve went ahead and did what Valve does best: create a sweet promotion that will raise awareness of its games and client, earning them big bucks in the long run. They have all sorts of fancy charts and graphs to prove this works, I'm sure.
Starting now and lasting until May 24, Portal is free on Steam. That goes for Windows and Mac, so there's no reason not to queue up the download right this second. Since I already own this phenomenal game, I can't say whether or not massive demand has the servers struggling to keep up. Let's hope not!
[Update: If you're unable to play Portal on Mac Steam, close it and run this in Safari: steam://run/52003 . This will open Steam bypassing the beta check.]
[Thanks, sicPOS, @Jcapote]