Here are just a couple that we have been discussing here at UNSW Built Environment over the last week as they pertain to city data and how we can overlay other data onto very approachable city/map tools in our browsers.
The RichBlocksPoorBlocks site brings together Google's Map interface and some fascinating US Income and Rental data that quickly reveals the various neighbourhoods in some big US Cities. I found the rings of relative affluence around many cities quite intriguing (See Texas for good examples).
Check the Twitter Tongues Map of London which beautifully shows the pockets of language speaking.Then came the Twitter NYC version which is just as nice.
[SimCity Images via SplitKick]