= new reply since forum marked as read
Oh, and atomic power plants and such.
Now I'm not going to get any work done this afternoon. 
Thanks!
Fission, the splitting of atomic nuclei larger than iron will convert a small amount of mass into energy. Some atoms are much easier to split than others; uranium 235, the atomic bomb stuff, is easily split. Some atomic isotopes are unstable and split spontaneously in a random way. The Curiousity rover on mars is powered by such an isotope, plutonium-238.
Radioisotope within a graphite shell
that goes into the generator wikipedia
Fusing atomic nuclei smaller than iron releases energy too. The fused nuclei have less mass than the original unfused nuclei. Fusing smaller nuclei to produce energy is difficult technology. It happens in stars like our sun, and is made to happen in nuclear fusion weapons (so-called "hydrogen" bombs) using the energy produced by a fission (so-called "atomic" bomb.
Neutrons, protons, and electrons have a specific mass.
Looking at the periodic table of elements, and examining the mass of individual stable non-radioactive atomic isotopes, there is a generally increasing but slight deficit in the mass one might expect considering the number of neutrons, protons, and electrons in the atoms as we move up the table to iron. Beyond the element iron, the expected masses begin to increase again.
In the bigger picture, mass *IS* energy, and energy *IS* mass. That's what that equals ( = ) sign means.
From a quantum physics perspective, even the emptiest parts of the universe are a stew of particles and anti-particles, such as electrons and positrons, neutrinos, etc., blinking into and out of existence in a random, statistical manner.
As one pumps more energy into a system, in the electromagnetic spectrum for example starting with infrared, moving up through the radio spectrum, through the microwave spectrum, visible light, ultraviolet, x-ray, gamma ray... one increases the odds that increasingly stable matter will appear in the quantum soup. That's the sort of thing studied at CERN, where physicists pump huge energies into very small systems by accelerating charged particles and crashing them into targets, creating various quantum stews of energy and particles.
The same sort of energy-mass relationships exist in ordinary chemical reactions too, the carbon and hydrocarbons in a fossil fuel, and the oxygen in the air loose a bit of mass when combined to make carbon dioxide and water and "energy," let's say in a power plant or automobile, but the change in mass is many magnitudes less than one might measure in nuclear fission or fusion reactions.
