Proxies

Using filtered proxies you can create simple sorting systems that are both fast and easy to understand. The filters on the right are black lists so all items that don't fit any other category can go there. (Faster and more efficient designs are possible)

Proxies can be used for transporting items more quickly than you otherwise could. That's can have as many hoppers as you want pull or push from a single block.

Note that the proxies still point to the furnace, because you are trying to pull from the furnace. For extracting items the direction of the arrows is the opposite of the item flow.

Unnecessarily complicated? Yes, but it's just an example.