The main theme isn't Halloween, but the atmosphere is really fitting. Has a button that turns people into skeletons.

Same as L4D, but free.

Kind of like Ghost Master, the difference being it's 2D and less gameplay. Still worth a try if you like how it looks.

A 2D platformer with a halloween theme.

Not your usual tower defense game. Has goofy zombies and awesome night levels.

Set in steampunk Eastern-Europe, the game's atmosphere feels just right for this season.

Probably don't have to introduce this one either. Has tons of user created content, just look around garrysmod.org under the halloween tag.

I'm putting this in instead of Gone Home. Similiarly good atmosphere, but with some actual gameplay.

You probably know what this is. Has been getting Halloween updates ever since relase.

Again, more useful for getting into the grimmer, lonelier mood of Autumn. An amazing RPG.