The Historical Marker Database (HMdb.org) is a voluntary effort to document and catalog historical markers, mainly from the US and Canada (a few markers from foreign countries are also listed). Markers are listed by category and state; you can also search by multiple parameters (keywords, postal code, county, etc.). Marker info pages have a photo of the marker, a copy of the marker text …
… latitude/longitude of the marker, a context photo showing the general surroundings of the marker, and a list of other nearby markers. A link will take you to a Google Maps page showing the location of the marker and other nearby markers; a link at the top of the map page will let you download a GPX file with all the marker positions for upload to your GPS unit (TomTom POI files are also available). There’s even a custom version of the page for mobile phone; enter your latitude/longitude and get a listing of up to 20 markers near your current location.
The site also accepts submissions of historical makers not yet in the database (requires free registration).