Haunt the House: Terrortown, now available on PC and Mac, is a side-scrolling scare 'em up, originally released for Playstation Mobile devices way back in January. It is, so Eurogamer's review tells me, a brief experience but it does look charming, even when the tiny people becomes so frightened that they plunge out of a window to their doom. I remember thinking I was the only person in the world who had the stubborn persistence to stick with Ghost Master until the bitter lack of an ending and I've hoped for more haunting games ever since. If we'd all played the ghosts in Pacman instead of the yellow eater of worlds, gaming would be far more interesting. There's an earlier browser version of Haunt the House to toy with - think of it as a demo - and the lovely soundtrack is available to purchase as well.
I'd quite like a multiplayer Knock Knock, with one person playing the part of the horrors.