If you pre-purchased or plan on pre-purchasing Left 4 Dead 2 on Steam, there's a spiffy little extra you'll get if you also play Valve's Team Fortress 2. Namely you get Bill's beret from Left 4 Dead 1.
But this isn't just some cheap marketing gimmick, nope. There's a backstory.
... while conducting a science experiment on parallel dimensions, Bill's hat fell off a table into the Team Fortress universe.
And there ya have it. I would have liked to see the Heavy try to wear Rochelle's pink shirt or the Spy wield the frying pan. I suppose this will do.