I was getting ready to write a scene the other day that involves the bad guys blowing up a building with gunpowder. And it occurred to me that I didn't have a clue what that would look like. So I Googled "gunpowder explosion" and found this really neat YouTube video that is one of an eight-part British television program called "The Gunpowder Plot--Exploding the Legend" in which they actually recreate the explosion planned by the famous would-be Gunpowder Plot assassins.
They go into all sorts of wonderful detail about early gunpowder and of course the dynamics of such explosions. And then we get to actually see the an explosion taking place, right before our eyes. A writer couldn't ask for anything more!