Why does adding salt make water boil more vigorously?

The reason for this is that the salt crystals (you can put sugar in it and do the same thing) act as what are called nucleation points. They make it much more easy for bubbles of the vapour which is the boiling bit of water at the bottom of the pan to form. It's hard for a bubble to form in the first place because of the water crushing in on it because of the surface tension if you like. Small bubbles are very hard to form and if you put crystals in then they disrupt the structure of the water, they make it much more energetically favourable for a bubble to pop into existence. That's why you get this intensification of the boiling point.