this.

Alphabet, so probably communications

the real one that is not listed is agricultural equipment because it got most of the population out of the fields so they could come up with and use the ones you listed

Transportation (cars,planes,trains,ect..)

without coolant and heating devices we would not be able to have sterile conditions through heat and would not be able to keep food fresh, thus wasting alot of products ( alot of food would be wasted, and efforts would be lost and we could never be at an economical state and forever be in economic crisis. People would die, we would waste energy and we would deplete our earth quicker than thought possible.

this, except for the economical crisis part.

the economy wouldnt be the same without them, so you cant say if there would be a crisis or not

Mankind has gotten along just fine with w/o heating and cooling devices

They're more of a luxury in my opinion

All of these are cool, but the bong is the best.

I would have to say that the most useful discovery (is this in the history of man?) would have to be crop rotation or selective breeding.

As for most useful invention, probably gunpowder.

Best invention ever: the oven