Update: The sale is over. We hope you managed to grab TF2 -- for yourself or a friend -- while the getting was good.
Original: Actually, it's less than two hours now, so what are you waiting for? It's two dollars and fifty cents! It's usually twenty whole dollars. Go buy it. Now. Go!
If you grab it now (or if you already have it), you can enjoy the first annual Team Fortress Haunted Halloween Special, which will bring new haunted maps, exploding pumpkins and update-specific Steam Achievements. We'd write more, but that would only waste more time. Time you could spend buying Team Fortress 2.
[Via TF2 Official Blog]