This idea is so incredibly simple and wonderful I can't believe it took me so long to hear about it. You don't need an air popper to make your own fat free popcorn.
What you need:
1/4 to 1/2 cup popping corn
A brown paper "lunch" bag
A microwave
Pour the popcorn into the bag. Fold the bag over a couple of times to seal it. Place the bag in the microwave on its side, distributing the kernels along the bottom. Cook on high for around two minutes or until there is a two second pause between pops. (Just like processed microwave popcorn.) Pour into a bowl and butter and season as desired.
The time that you could it might vary per your microwave and the popcorn might burn so watch it while it pops. Unpopped kernels can be thrown back into the bag to be popped again if you wish.
There are only 30 calories in a cup of popped plain popcorn. Popping corn and lunch bags are way cheaper than even generic microwave popcorn. You won't get any harmful chemicals (from the bags) or preservatives and junk in your own microwave popcorn.