Maybe not the creepiest, but what's on my mind right now...
I just moved into this house a year ago. There are child-sized handprints on one of my bathroom walls. Permanent handprints you can only see from a certain angle, like what water exposure does to paint. The previous owners have no idea what I'm talking about, and I don't know how they got there.
EDIT: I've been told I should put this here.
EDIT 2: Many of you have suggested grease or oil from kids' hands, which are reasonable explanations, but my room and bathroom are in the attic, which was only finished and turned into a suite by the last (childless) owners. I don't think many children, if any, have been up here. That's not to say I think it's a ghost or anything, just that it would take some odd circumstances for that to occur. I should have mentioned it before, sorry!
EDIT 3: First off, I had no idea so many people would comment! Awesome! Some people wanted a size comparison, so I took a (shitty) second picture to try and satisfy them. I think I figured out what bugs me about the prints: they're child-sized, but not child-shaped. Well, that and they exist at all. Here's the pic. You can see the measuring tape, which puts them at around 4 inches, and my thumb for reference. My hands are small (my friends say I have carnie hands), so these prints are pretty petite.