This, from University of North Carolina Political Science prof Frank Baumgartner:
I've compiled a geo-coded database of every execution since 1976? Slides 17-19 on this slide show give some overview of the data.
He is co-author of The Decline of the Death Penalty and the Discovery of Innocence. More on the book, here. You can order it from Amazon.
Still more from Baumgartner, here.