Any flat surface in the house! It makes me absolutely crazy. Granted, I tend to drop things myself at times but then I do go pick up after myself and somewhat deal with the papers, etc. DH on the other hand.... you get the picture. He's like an 8 yr old boy with pockets full of snails and puppy dog tails. It's never ending. And, like zozee, why is the used paper towel still lying on the kitchen counter next to the garbage can?!?
I try to keep the clutter in the public areas of the house down to a dull roar on a regular basis. The upstairs office (my domain) and the sewing room (again, mine) are a different story. I get to those areas when I feel like dealing with them. Or when I can't see the top of the desk or the cutting table, lol.