Valve has been teasing fans with a Halloween-themed update for Team Fortress 2 for a couple of weeks, and now it's here. Just in time for October 31 too! That was probably intentional, eh?
The "Scream Fortress" update adds tricks and treats alike, bolstering matches with random gift drops and loot all this week. Each of the game's classes also get new costumes to run around in, and you can apparently expect to see a giant skull and an "enormous haunted eyeball" as you play. Also, new Achievements to unlock. Yay Achievements. You can then top it all off with a free tie-in digital comic, available on the Team Fortress website.
Source: Team Fortress 2 website