I have also seen downright wing-style spoilers in the front hood of a few cars, but those are rare, especially in the real world of motorsports.
Cars like the 360 Modena dont have external, visible spoilers, because the whole under body panel is designed to handle the job of downforce and avoiding drag.
BMW 5-Series and 3 Series have specially shaped gas tanks that act like an upside down airplane wing to bring down the back end under high speeds, to increase stability on the highway, incase of the need for an evasive maneuver.
Excluding race cars, any street car could get enough downforce from a specially designed under body panel. There really isn't much need for a visible spoiler on the top of the car.
The Ferrari Enzo also has extensive dynamics under the car to give it amazing downforce, and to also provide a smooth surface for air to run past, as opposed to the un-aerodynamic parts exposed on normal cars. i.e. Frame, exhaust, mufflers etc.