Goodness, that happened with absolutely no fanfare. The original Portal, if you somehow don't already own it, can now be installed and played for free via Steam. I've just checked it on a spare Steam account and it works just dandy. This is true of both the PC and the Mac version, by the way. If you're determined to pay money for it, you can still cough up for The Orange Box or the Portal 1+2 package, but just Portal itself now defiantly costs no-pennies. Grab it from here.
Update: transpires that this is only available until September 20th, as part of a games and learning initiative from Valve. So get your skates on, yes? As long as you install the game before the expiry date, it's yours to keep forever.
You can see more on Valve's "Learn With Portals" program, wherein they're encouraging kids to create Portal levels themselves, in the rather charming video below.
(Big thanks to all who mailed about this)