While arguably probably not my best work, it only took me all of a couple hours, I present a listing of all the Texas Historical Markers. I don't know why I never linked to it before. Maybe I'm not too proud of it, but I wanted to give you access to it. What features do I need to add?
I discovered one evening that Texas has a database of all historical markers in the state freely available online in a comma separated value file (among other formats). What's a geek to do but grab the file and throw it in a MySQL database!
I whipped up a quick drop-down list of all the marker names and used AJAX to show the marker's text. A couple of simple URLs allow you to see, generally, where the marker is located.
I have the location information in the database, but it's not Lat/Long which would make for easy map-making. Perhaps that will be my next step. Found a supporting .txt file that has most of the Lat/Long. A simple JOIN from the database fixed that problem. (though, to be honest, some of the coordinates are way off. Unless Texas has markers in Mexico.)
At any rate, take it or leave it, there it is: http://n5dux.com/histmark/