Historical Mysteries: Selected articles from the Summer 1993 issue of Mystery Readers Journal.
The Detective in the Toga: Website dedicated to mysteries set in ancient Rome.
The History of Mysteries: Website dedicated to explaining the evolution of the mystery genre, and the sleuths who figure in them.
Historical Mystery Appreciation Society: Like the name says. You can join to receive their quarterly newsletter and/or volunteer to assist in production of the newsletter.
Soon's Historical Fiction Site: The scope of this site is much broader than just mysteries. It also includes a master list of all historical fiction authors, and authors sorted by time period/genre, and many, many, many more links.