V-Cube 6 (6x6x6)
The V-Cubes are based on patents secured by Greek inventor, Panagiotis Verdes. The patents cover mechanisms for cubes ranging from 2x2x2 to 11x11x11 although products have only been released in the 5 - 7 range.
The V-Cube 6 was the first mass-produced cube with these dimensions. Unlike the odd-numbered V cubes, the V-Cube 6 makes a click as each face is turned - if you don't hear the click, you haven't quite lined the faces up properly and it might stick on the next turn.
It's an expensive product - around £30 or more, even second-hand. It's a lovely thing to own and solve but easy to work out how to do when you scale up the method you use for the Revenge or Professor's Cube.