All estimates are still guesstimates... Many think it's between 500 - 1150 Tons of TNT. Who really knows the truth. Was it really only the Ammonium Nitrate that exploded? Doesn't seem to be that from the severity of the explosion.
Hezbollah are the masters in using Ammonium Nitratrates as explosives, though they quickly denied that they did not have arms cached in there.
NASA just released a photograph from their ARIA (Advanced Rapid Imaging Analysis) team.