np: Biffy Clyro - Shock Shock
I'm going to assume this doesn't Abilities like Adaptability or Technician into account, am I right? Still this is a very useful tool.
But it does take into account Thick Fat, Wonder Guard, Solid Rock, Filter, Volt Absorb, Water Absorb, Flash Fire, Levitate, Heat Proof and Dry Skin when calculating the damage against Pokemon that have them. It also takes into account the ratio of Pokemon having one ability or the other (for example, it takes into account that there are more Levitate Bronzong than Heat Proof ones).