Most physicists familiar with special relativity know that in it, the energy E and momentum p of a freely moving body are related by the equation E2 − p2c2 = m2c4 where m is the mass of the body. Alas, not all of them realize that this formula is incompatible with E = mc2. But an even smaller number of people know that it is perfectly compatible with E0 = mc2, because E0 is the value assumed by E when p = 0. This article is written for those who do not want to be lost in three pines1 of the above three formulas and who wish to attain a better understanding of relativity theory and its history.
There's a very nice history leading to the development of the equation, and beyond. A recommended reading.