I use a Logitech USB Marble Mouse, optical trackball.
http://www.logitech.com/index.cfm/products/details/CA/EN,CRID=2150,CONTENTID=5003
And yourselves?
http://www.logitech.com/index.cfm/products/details/CA/EN,CRID=2150,CONTENTID=5003
And yourselves?