You might also be interested in The Best Resources About Wealth & Income Inequality.
Here are my choices for The Best Sites For Learning About Taxes:
What America Pays In Taxes is from NPR.
No Taxation Without Calculation: Filling Out Tax Returns is from The New York Times Learning Network.
Who pays more in taxes? is an interactive from The Washington Post.
The 1040 Form Turns 100: Resources To Explain Income Taxes is from The ASIDE blog.
Tic Tac Taxes is from EconedLink.
The state of the American tax system, in 8 charts is from The Washington Post.
Additional suggestions are welcome!