its called holonomic drive. they use a set of four indipendently driven - angularly oposing wheels that have roleres on them that allow the wheels to slip sideways. or in the case of mecanum wheels diagonally.
there is also swerve AKA Crab Drive it uses wheels mounted on independntly steered modules that can be steered in 360 degres.
see
http://www.chiefdelphi.com/media/photos/tags/mecanum
http://www.chiefdelphi.com/media/photos/tags/swerve
http://www.chiefdelphi.com/media/photos/tags/holonomic