You see them around, although when I signed up I found a 'hack' that showed a list of the possible corporate coupon codes you could use and I iterated through to find the ones with the best terms (this was a few years ago)

You certainly don't want to sign up for ZipCar without a corporate discount code. Not all of them require you to be an employee.

I own a car but keep a zipcar card in my wallet. There is a massive feeling of security knowing in an emergency/etc I can whip out my smartphone, book a car nearby and be driving within a few minutes.