Well, in nuclei there are positive and neutral particles together. To keep the nucleus from bursting the strong nuclear force (attraction) overcomes the electric repulsion. Due to the physics, there is more energy "stored" in a nucleus than in its free constituents. This appears as a higher mass. In a nuclear bomb for example, the net mass of the products (assuming we can gather them) is slightly less than the net mass of reactants. This is due to the change in magnitude of nuclear forces in the nuclei. The mass difference is the energy produced in the explosion.
Another way is by creating antimatter (via some reactions, say beta-plus decay). When an antiparticle meets its matter counterpart, both annihilate and turn into radiation (gamma photons). Also, a photon can materialise into a matter-antimatter particle pair.
Quote (DrDrugs @ 14 May 2011 07:22)
They are working on a fusion reactor near San Diego, they are close to getting it online, It takes them like d days to create the molecularly perfect gold targets thy shoot with those lasers, looks promising.
It's not really close. They still have serious troubles maintaining fusion and actually getting more power out than they put in (also the efficiency needs to be increased, no use to have the amount of energy put in to be as much as 99% of the output).
This post was edited by sevlo on Jun 5 2011 10:39am