Check out Hotel Chatter's list of the 10 grossest things they've found in a hotel room. A big eww to all 10.
I think the worst I ever had was a big blood spatter on the wall and part of the ceiling, partially hidden by the curtain. I mean really big, like someone gutted a moose in there (or so I told myself while running for the front desk). I also found what looked suspiciously like a hairball coughed up by a cat in the tub once.
Semi-related: About That Item You Left in Your Room...
OK, your turn.