I used to reinstall Team Fortress 2 every year for Halloween, dressing up in seasonal cosmetics to fight giant enemies like the Horseless Headless Horsemann, Monoculus, and Merasmus, or to just drive around in karts. Those events continue, and Scream Fortress 11 is live right now. As is the case in recent years it's more of a showcase for player-made creations, with two community maps and a host of community-created cosmetics.
Those maps are a king of the hill map called Laughter, and a payload map called Precipice. The cosmetics are bundled in a collection called Spooky Spoils which includes a bird head for the Engineer, a shark head for the Pyro, and a two-piece velociraptor costume for the Scout. The Soul Gargoyle from previous years returns as well.
Scream Fortress 11 continues through November 7, and you can find Halloween matches to join under the Special Events category of the Casual section.