Federation of Tax Administrators - This website provides tables comparing the tax rates and structure for all fifty U.S. states. State income, sales, and excise taxes are covered. It also compares related items, such as check-off contribution and tax amnesty programs. Presentations of state and local tax burdens include revenues by percentage of personal income, per capita, by source, and as a percentage of total revenues.
Internal Revenue Service - This site from the Internal Revenue Service includes all federal tax forms, publications, instructions, and regulations, back to 1990. Also includes information about filing extensions.