The Ellingham diagram plots1 the standard free energy of a reaction as a function of temperature. Originally the values were plotted for oxidation and sulphidation reactions for a series of metals, relevant to the extraction of metals from their ores (extraction metallurgy). These reactions generally involve the reaction of a gaseous phase (the oxidising gas) with (almost) pure condensed phases (the metal and oxidised compounds).
By using the diagram the standard free energy change for any included reaction can be found at any temperature. Along with allowing you to calculate the equilibrium composition of the system, the data on the diagram is useful in other ways, as we shall see.
Note: Due to shortage of reliable experimental data, some of these lines are not plotted over the complete range of temperature.