The NUKEMAP by Alex Wellerstein is probably the most fascinating interactive map I've seen in years, and certainly the coolest thing I've discovered in 2015.

Curious what would happen if the weapon used against Nagasaki (the 21kt Fat Man) were instead deployed on downtown Pittsburgh?

Fat Man if exploded over downtown Pittsburgh

How about the largest nuclear device ever exploded (the 50Mt Tsar Bomba) being detonated in London instead of the Arctic Circle?

Tsar Bomba if exploded over London

The big limitation to the NUKEMAP is terrain awareness. Simulating terrain would take hilarious amounts of data and computation not feasible for a browser, but would produce even more interesting results. For example, even though the bombing of Nagasaki used a more advanced weapon design with a higher yield, there were fewer casualties when compared to Hiroshima due to the hilly landscape. (Well, that and bad aim due to poor visibility from previous conventional bombings nearby.)

Still, tread carefully, I've already lost several hours to this site.