One of the most unusual airliners of all time had to be an East German four-engined airliner, of which two prototypes were built by VEB in the early 1960s. This oddball had two two-wheel main bogies in tandem under the fuselage, much like a Boeing B-52, with smaller struts that retracted into each of the underwing engine pods, as on the Il-18 Beagle.
(Dis)honorable mention to the following:
(1) The flying car designs, such as the ConVairCar and a prototype involving the wings and tail of a Cessna Skymaster mated to a Ford Pinto.
(2) The Convair Pogo, with wheels on its tail; this aeronautical wonder sat on its horizontal and vertical stabilizers with its nose pointed straight up, much like a rocket. As for accomplishing its goal of being able to take off without a runway, the plane performed admirably. The problem was how to land the thing in one piece....
The aeroplane has unveiled for us the true face of the earth. -Antoine de Saint-Exupery