Urban-Brookings Tax Policy Center
This site is run by the Urban Institute and the Brookings Institution
and it deals with all sort of tax issues. There is a great deal of
data and analysis (the latter both formal and informal). The emphasis
is on federal taxes, but there is still a fair amount of information
on state and local taxes. Both those involved in public debates and
those teaching public finance will find it very useful.
http://www.taxpolicycenter.org/