You get a small or medium sized car for £9.99 for one day hire. Great if you need to get to the airport as they are covered.
Dates are available between 14th December 2016 to 23rd December 2016 .
It states that deal starts tomorrow but I was just able to book successfully now for Brighton to London City Airport on 14th December.
EDIT: Link not working - to view deal on site click Offers then UK Offers
EDIT 2: geeadamg mentioned Europcar's deal
which is still running, only £1! Doesn't go direct from airports but I have worked out that by using this deal I can save money and deal with getting my luggage to my doorstep for my return journey, although it does mean I still have to journey to drop the car to it's destination I can do so sans luggage and sans passenger so it's going to help me get back from the airport!