Cone Spanners
Essential for overhauling hubs. You need a cone spanner (along with a normal spanner) in order to lock in the cone adjustment after overhauling a hub; if you don't you run the risk of the cones tightening as you ride and possibly destroying your hub. A cone spanner is basically just a very thin spanner, thin enough to fit in the narrow space available for them to fit on the hub cone.