I have a wheeled Rimowa Salsa "Cabin trolley IATA"

http://www.rimowa.com/index_int.asp?spr=Englisch

55 x 40 x 19.5 cm and fits in the gauges. Comes in silver, black, and a variety of other colours.

Made of polycarbonate, so you seem to be able to keep on stuffing things in and in practice it can stretch more or less up to the max thickness of 25cm (overfilling it doesn't cause it to stretch lengthways so it still stays within the 56cm). Empty, it weighs practically nothing. Indestructible, so doesn't mind being checked in if needed (sometimes I do, sometimes I don't).

Only disadvantage is lack of outside pockets for quick access to stuff. But if I travel with a coat with several pockets, that isn't an issue.

(This is also the model Lufthansa sells as own-brand cabin baggage with its logo on.)