Steam is running a massive Halloween sale that ends October 31. A total of 100 games/compilations/DLC packs are discounted up to 75% during the promotion.
Some recent or noteworthy games included are:
Hell Yeah! Wrath of the Dead Rabbit - 50% off
Fallout 3: Game of the Year Edition - 50% off
Amnesia: The Dark Descent - 75% off
Alice: Madness Returns - 75% off
Dead Space 2 - 75% off
Bioshock 2 - 75% off
Limbo - 75% off
Left 4 Dead 2 - 75% off
Plants vs. Zombies Game of the Year Edition - 75% off
Dead Island - 50% off
Darkness II - 75% off
Head over to the Steam Halloween sale, to buy some scary games.