Porn (naturaly).
Booggers wiped on the front of the drivers seat.
Used tampons and maxi pads under the drivers seat.
Dried tobbaco/spit juice in the pasengers door pocket.
"Marital aids" consisting of whips, chains, masks, hoods.
Polaroid pictures of naked people in compromising positions.
A cattle prod.
A cigarette pack in an oil pan of a new car.
Beer can in a saeled fender of a new car.
Coke board and disguises in a car that I checked out after it was used in a bank robbery.
All of the above were from when I worked as a line tech for a major luxury car dealership.
In my own cars the only odd thing that I remember finding was a mouse skeleton under the carpet of my Spitfire.