Campfire is a plugin which allows you to have campfires in Minecraft in which you can burn tour minerals and logs to get the same result as burning them in a furnace would give.
How it works
Putting fire above or beneath a supported block, or putting a supported block above fire, will make it turn into the result you would have by burning it in a furnace.
You can also make a piston machine with pushes the supported block above fire to make it drop the item. If you are using a furnace, the resulting item of the burning will always drop to the pistons right hand side, 2 blocks in front of the piston and 1 block to the right.
Note: Putting fire on top of Netherrack to make it burn into Nether Brick is not longer supported as of version 2.9.1. This is because it prohibited the ability to use Netherrack as a block for eternal fire. It can still be burnt the other ways, with, for example, pistons or placing fire underneath it.
Burnt Block Resulting Item
Gold Ore -> Gold Ingot
Iron Ore -> Iron Ingot
Logs (all types) -> Charcoal
Clay -> Hard Clay
You can, as of version 3.0.0, add your own burning results to config.yml!
Plugin Demonstration (old, but still applies)
Plugin Demonstration in German by MineCraftler4Live
Featured in this video by TwixPvP