A fire rod to cook a cow alive!
A decent weapon that is also quite cheap but it cannot stack. Hitting a mob will set it on fire for a short time. The fire rod has one use and cannot stack.
It is sometimes possible to damage players with the fire rod in SMP servers where PVP is not allowed. It remains untested exactly how the fire rod circumvents this restriction, or in which cases it is able to.
The Fire Rod does the same damage as if you would hit the mob with bare hands, meaning it does 1 Damage (1/2 Heart). But it also lights the mob on fire, dealing 1 Damage (1/2 Heart) per second for a random amount of time.