Food grade diatomaceous earth.
It can be a bit messy, because it's a fine powder like flour or pixie dust so like others have said, a mask is a good idea.
However, DE is all natural, edible, and it works. (get the food grade DE so you know it is not treated with a chemical like the DE for pool filters)
You can rub DE right onto the animals skin and fur. (Yes, they will shake right after you do this, so applying DE outside might help the mess factor)
You can sprinkle DE into your carpet and pet bedding. Let sit for a day or so to kill adult fleas. Then vacuum and dust...for a few days
It's very handy stuff to have around the house and garden.