The Tax Policy Center is an invaluable resource for researching tax policy. It is a non-partisan organization, founded by alumni of the Clinton, Reagan, and Bush (the first), administrations.
The Center on Budget and Policy Priorities (CBPP) is a think tank focused on tax and fiscal policy. They provide in-depth analysis of state issues.
Citizens for Tax Justice is an organization that represents low and middle income citizens in the tax debates on Capitol Hill.