Furnace

The furnace is a special block currently only available in the indev, infdev, and alpha versions of Minecraft. Before the furnace appeared, smelting was accomplished by creating a fire and dropping the ores into it. Now the furnace has its own smelting menu where several smelting operations can be done. It consists of one field for the object that will be smelted, one field for the output object and one field for a fuel. The fuel used can be either log, plank, stick, or coal (plus chests, workbenches and fences neither of which are very efficient).

Each operation takes 10 seconds.

A pickaxe is required to pick up the furnace. When the furnace has items inside it, do not try to pick it up, because it will destroy the items inside.

The furnace will take time to smelt objects, but it will also work when the smelting menu is closed, as long as there are still objects to smelt and there's enough fuel. If all objects are smelted, the furnace will stop using fuel. When the fuel runs out, the heat in the furnace, indicated by the flame icon, will slowly decrease and when it is gone, the furnace will become inactive. If a smelting process was running, it will be aborted and has to be started again from the beginning.

As a building material
Furnaces take more time to destroy than other blocks, making them a very valuable building material. However, since they require 8 blocks to craft, they are tedious to amass in any large quantities.

Tips and tricks
8 coal will smelt an entire stack of 64 blocks. 43 planks are the next best thing - 43 planks are capable of generating 64.5 smelted blocks. If you want to be really efficient and not lose that half-plank to nothing, put in 44 planks, then take the first two smelted blocks out, and put two extra unsmelted blocks in.

Remember that meat (like other food items) won't stack. This means that 1 coal will fuel between 6 and 7 units of meat, depending on how fast you are with re-loading the furnace. Sticks come in handy, as two sticks should be enough to cook an eighth unit of meat without too much fuel left over.

The bonus of using planks and sticks is that they're a renewable resource, unlike coal.