A magnetic crane like the ones used at scrapyards would work.
Cut the current and the car will drop.

But for what you're trying to accomplish, why not do it like
all the other prom night demonstrations? Get a car that was
involved in a DUI from the local junkyard. If it still has
the beer cans inside, it's that much more realistic.

TJH wrote:
OK, perhaps this might sound strange, but I'd like to drop a car from a
crane.

I'm thinking about doing a project for a high school senior class to
show the violence of an auto crash (educational for prom week) at
roughly 30-45mph. In order to do this, I'd hoist an auto by a crane up
say 40 - 50 feet, and then drop it to the ground. But, to do this, I
need some type of quick release mechanism, because the crane owner
isn't going to risk damaging his equipment by free-falling the cable.