Tutorials/Automatic smelting

Semi-Automatic Furnace using Hopper
You will need 3 Chests, 3 Hoppers, and 1 Furnace. To make these, you will need 8 cobblestone, 15 iron ingots, and 12 logs of wood (48 wooden planks).

Build instructions
Follow the schematic from the ground up. Sneak (hold down ) to attach blocks to each block that has an inventory:
 * Step 1: place down a chest on the ground. (This will be the chest in which the smelted / cooked items will be put)
 * Step 2: sneak (hold down ) and attach a hopper to the chest (side or top is fine).
 * Step 3: sneak and place the furnace on top of the hopper.
 * Step 4: sneak and place one hopper on the top and 1 hopper on the left side (important) of the furnace. (The top one will insert the items into the furnace, which get smelted / cooked; The hopper on the left side inputs coal into the furnace)
 * Step 5: sneak and place a chest on top of each hopper attached to the furnace.

Note: by using a hopper to empty the furnace, you will forfeit any experience points from the smelting.

Variations: you may choose to add Switches to control the hoppers. Switching off the bottom hopper stops it from taking items out, allowing you to collect the experience from smelting them. Switching off the top hopper allows you to fill the top chest with various items, then eventually smelt them all in one go, reducing the fuel waste normally incurred by smelting partial stacks. The fuel hopper on the side need never be switched off.

Videos
A video on the process.

Updated Tutorial:

Tiny Charcoal Generator
First appearance: April 1, 2013 from this video.

This lets a furnace producing charcoal feed its product back into the furnace to fuel itself. It needs to be started off with one piece of charcoal (or for that matter, any fuel) in the chest. If more than 5-6 stacks of wood are available, another chest for input can be placed atop the upper hopper. Note that this design uses a comparator, which implies access to Nether Quartz.

The chest will not start receiving charcoal until both the furnace and the hopper feeding it fuel (level 2) are full. However, the hopper can be "stuffed" with four non-fuel items in its last four slots, as long as you leave the first slot open for fuel. Then the chest will start getting charcoal after two stacks of charcoal are produced, which will require 2-$1/4$ stacks of wood.

As usual, using a hopper to take product from a furnace forfeits any experience from the smelting.