Omni wheel

From Combat Robot

Omni wheels, similar to mecanum wheels, are wheels with small discs around the circumference which are perpendicular to the rolling direction. The effect is that the wheel will roll with full force, but will also slide laterally with great ease. It is a method of creating holonomic drive.

They are often used in small robots. In leagues such as Robocup, many robots use these wheels to have the ability to move in all directions.