Just a bump post...
One of my old favourites wasn't in a dungeon, but in a lonely farmhouse. A young barmaid lived there after her parents died. The PCs interacted with her a few times. After many games, the PCs were asked to check up on her because she hadn't arrived for work for two days.
So the PCs get there and (for some reason or another) think everything's normal. They open the door after knocking on it and getting no response. It's dark inside, the shutters being closed, so they go to open a window. One failed Balance check later and one of the PCs is on the floor, having slipped in a pool of blood. They look up and see a large stain on the ceiling, and the still-steady drip-drop of blood.
They rush upstairs and find the barmaid strung up, half-naked, by her ankles in a small study. There is a large cut running from her neck to her stomach, and blood is still slowly dripping out of it. Two PCs move in and cut her down (while making Balance checks). The Wizard PC secretly makes a Will save, and fails. He plays this out by having the Wizard gag in the corner at the sight, trying to compose himself.
The lone PC waits in the study while the other two rush downstairs to try and save the barmaid, leaving the Wizard alone. Something small and black as deepest night, about the size of a cat, rolls out from under a table. It unfurls its wings and streches out, and streches out, until it stands 5 feet tall. It smiles at the charmed and fascinated Wizard and clicks its claws together. The Wizard's faithful owl familiar screeches, and the other two PCs arrive to fight it.
A very cool battle began. The two PCs went down, and the Wizard remained in a trance. The beast approached the Wizard for a Coup de Grace, but once again the owl familiar saved the day. It swooped in and attacked, and while it did no damage, this action roused the Wizard. The Wizard was able to drive off the wounded beast just in time to watch one of his friends bleed to death...
...or not, actually, but that's another story.
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Then there was the "Old Well" that housed a Cthulhu-ish monstrosity, full of tentacles, Improved Grab, and a Str-draining attack.
The PCs descend down the well and find a small tunnel leading to a natural cavern. In the cavern they find the skin of the monster's victims all stacked in a pile. In crude, child-like writing, the monster has written his journal.
Various other curios lie around the cavern, momentos from the monster's victims, and a heap of bones in one corner. Some of the bones are human, other animal - including a bear skeleton.
Anyway, when the PCs are finished looking around, they hear the monster return home...