By far my most favorite computer game in the world is Phantasmagoria! I first played it when I was 12 and it scared the bejeebus out of me! It all had to do with exploring your newly bought mansion (the grounds and the local village), unleashing a dormant demon, and fighting for the life of your husband. It came out in 1995 and was absolutely cutting edge. I’m sure its graphics are a little weak compared to today’s standards, but I still think it’s way cool.
If you like your games a little more light-hearted, here are several fun games you can play online:
T. F. Powell's Haunted Castle
3D Trick or Treat Adventure
The Ghostly Grounds of Gargor
Animated Halloween
Caverns of Blood
Halloween Flash Games @ One More Level
*Flash is required for some of these games. You can get that here.