Don't Starve, since it generally involves a hefty amount of eating (candy) and more mad scientists than any other festive day. To celebrate, Don't Starve is half off on Steam, $7.50, and a horror mod called The Screecher is free to all owners.
The Screecher turns the Don't Starve design on its head, offering players a view of the main character in third-person, from behind, rather than the front-facing angle in the main title. It's less of a "build things to survive" game and more of a "find dead bodies and blood in a dark forest and run away from the terrifying monster to survive" game. Think Slender Man meets Telltale's The Walking Dead with developer Klei's sensibilities.
Yeah, it feels like Halloween now.