Wolfram Alpha is a unique search engine that dynamically converts searches into graphical and tabular data.
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The "Examples by Topic" section shows the range of data it can compute. When searching, be sure to check out the source information link at the bottom of the page to see where the data came from.
Google Public Data provides visual representations of U.S. data at the national, state and county levels.
GapMinder displays data from 430 social indicators (demographics, heath, environment and much more) in graphs that "move" the data over time periods. Either browse data or use search box in upper right-hand corner.
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Example: "Changes in Life Expectancy 1801-2007 by Country and Income"