Taxes are a primary tool for redistributing income from the wealthy to lower income earners. The economic impact of the redistribution of income is a highly debated topic and arguments are generally formed in the Tax Reduction/Increase debates.
The following two sites look at world income distribution and provide a comparison over time
And:
http://www.imf.org/external/pubs/ft/weo/2007/02/
Data for the U.S. are also available from the Census Bureau at: http://www.census.gov Search for income distribution. From the Heritage Foundation, a conservative think tank, here is an article that addresses the impact of tax changes:
http://www.heritage.org/taxes/report/the-correct-way-measure-the-revenue-impact-changes-tax-rates