The Lincoln Institute says they’ve started a program to have George Washington University build a national database of real estate tax information. There’s a bit of information on GWU’s site, indicating that
a data collection team is compiling and classifying a wide range of material that characterizes property tax structures and processes in all fifty states to produce a “Compendium of State Property Tax Regimes.” The compendium will be available as a data set, and researchers will be able to perform simple queries through an interactive web site.
Lincoln’s 2007-8 program document is behind here but you’ll need to register, or use bugmenot.