From now until September 20, the PC and Mac versions of Valve's classic first-person puzzler Portal are free. Free gratis. You won't pay a cent for it.
A website has gone live (perhaps a little early) promoting the offer, which directs users to Valve's Steam service, where they're directed to download the game free of charge.
Not that this will be terribly useful to that many of you, since by now, four years on, surely everyone has a copy, but just in case you don't, have at it!
Learn With Portals [Valve]