Simon,
these are vibrators used in machinery handling sand, pebbles, that kind of thing, where vibrating and bouncing the medium around helps it to flow.
These particular ones use compressed air to turn them, the turbine wheel inside has some holes so that it is out of balance and creates a significant rotating force that can shake a machine or duct conveying material when mounted on the outside.
These are available in various sizes ranging from a GT4 up to a GT48.
Personally, I'm usually trying to stop things vibrating, so I don't use them:laugh:.
Dave