To celebrate Halloween, say Valve, they're making Left 4 Dead 2 free this weekend. And by weekend, they mean from today (Thursday) until Monday. And by Halloween, they mean stopping days short of Halloween. Ladies and gentlemen: Valve Time.
This means by following this link (and yes, this does use the steam:// protocol that Valve never found time to respond to) you can install L4D2 on your Steam account, and it'll be there, being free and yours to play, until Monday morning.
Current players can add a new achievement (weee) if they help a newbie survive a map over the weekend, hopefully encouraging everyone to get along and not have regular players throw a TF2 weekend style tantrum about it. Fancy a weekend with the zombies?