I came across Factor Samurai the other day while I was reading Flipboard. I've enjoyed playing this app and am excited to have my students try this out after winter break!
This is very similar to Fruit Ninja, but students will have to "slash" composite numbers down to their prime factors. You can only "slash" composites--if you slash primes you are penalized. There are 3 different challenging levels with different speeds. See my full review here in my app review. For being a free app, it is very cool!