I have just found a very interesting site called the Historical Marker Database. Simply put, volunteers add information on historical markers they come across in their travels. After editing, each entry can be searched and viewed. Some of the information that can be entered includes: the inscription, location by street and lat/long, pictures, and commentary. There are already quite a few entries in the database. But when you think of all the markers out there, this could grow to be something very interesting for the armchair traveler.
It could also be a fun tool for history teachers to get their students interested in local events or class trips. This is definitely worth a look!