I have used diatomaceous earth now for a couple of years. It is incredible and safe (as long as you don't use the pool-grade stuff), and works.
I have used it for fleas by just applying it on a wood floor and sweeping it to get it in the cracks and along the baseboard. I also applied it gently to one of my cats.
Believe it or not, I also used this particular DE product to get rid of bedbugs by applying it along the baseboards in all 3 bedrooms. Bedbugs gone - permanently. I used a really small paintbrush to apply and spread it (it was time-consuming and difficult - furniture had to be moved, etc. but it was worth it), and I shaked it as little as possible to prevent any dust flying.
It also got rid of an ant problem I had - I hate killing anything, but I had no choice and over the course of several days the ants diminished until they were completely gone. I applied it in 2 places - where they were coming in my house, and the threshold to my kitchen that they were crossing over.
Fleas, Bed Bugs
Diatomaceous earth is amazing! It works wonders as a dust bath for fleas on my dogs and I even had a friend that used it for bed bugs! Bed bugs is apparently a huge epidemic right now and so I thought I would share what my friend learned from her experience. As everyone has suggested make sure you use food grade d.e. but also make sure that it is in a powdered granulation... Not fine! And most importantly she was told not to put it right on top of the matress but rather to put it in the cracks and crevices, in the boxspring, on the headboard, etc. Where the bed bugs will hide!