Pololu hub to Lego Axle Connector
This allows a Lego axle to connect to a Pololu universal hub for use with their D-shaft gear motors. Link to universal hub http://www.pololu.com/product/1081
You could also use it to connect a Pololu wheel to a lego axle connected to a Lego motor as well.
The Lego axle hole may or may not be tight enough. Some PLA plastics printed a little