Policy Map is an interesting website. They have some maps that are widgets - so you can copy and paste the html into your website. For instance this is a map of unemployment in the US by county:
This is a cool google map of light pollution.
This map does a good job of showing the types of countries that use the death penalty and those that don't.
The National Broadband Map has some neat maps. For instance the speed test one shows that real speeds are slower than advertised ones in my area (but surprisingly in some areas they are actually slower than real speed).
There is an amazing website Urban Research Maps that lets you see how racial demographics have changed on a block level over the past ten years - between the 2000 and 2010 census.
It has data for only part of the United States, but this data includes PA, NY and several other major states.
My neighborhood in 2000
ArcGis Online has some neat maps like
this one which fits Americans into nice marketing categories. You can zoom in to the block group level (Census data category) and get detailed information about your community!
SoilKitchen.org organized free lead tests for a week. They tested over 350 samples, primarily from Philadelphia.
About 10% of the samples had > 1000 ppm lead (the point at which it becomes not so good for growing vegetables).
I color coded the markers based on lead levels and created this map:
The government has an interesting Food Environment Atlas that lets you map things by county.
One neat layer is pounds of fruits/veggies eaten at home per capita. They also have grocery store access, food prices, and a lot more!
The NY Times has come up with a new map using Census data on income, race, housing, same sex partners, and education. Nifty interface!
The data is from 2005-2009, the American Community Survey, which is happens in-between the major decennial census.
They display the data at the census tract level if you zoom in. I cannot tell if they are trying to show block level data - as they have dots representing data on blocs, however I don't think the American Community Survey does blocks.
Income, race, education, health, political participation and more -- all presented in a user friendly format.
The only downside is that their smallest area is a Congressional District. They should have done zip codes! More work, but so much more logical.