Head to itch.io's free games section filter by 'most popular' and enjoy a pick 'n mix of li'l horror treats. Some, like The Emulator, are the product of scarily themed game jams. Others, like 11:45 A Vivid Life, just have a weird, cool premise: "A short weird tale about a girl who discovered her skeleton isn't hers".
There are plenty of low-poly haunted house games in there too, but in a good horror game the atmosphere matters more than the assets. Also check out check out Midnight Scenes from Octavi Navarro, the artist on Thimbleweed Park.
Of course it's possible that none of these intentionally scary free games can match the arse-tightening tension of Hot Hot Panic, a game about trying to hold a conversation while stuffing your face with meat and potatoes.