Supply and Demand Chain Executive has created a subject-based category of solution vendors and consulting firms in the area of supply chain. The directory, from graphical design point of view, has been developed nicely and in the shape of a wheel.

The directory covers areas such as sourcing, procurement, Fulfillment, Logistics and Payment and though it only covers a certain number of companies, it's good as a reference.

You can see this Supply Chain Wheel here.