Cassini – An overlay of 18th century maps over Google Maps that lets you adjust the transparency of either layer. (I’ve got copies of the IGM maps of Central Italy from the early 20th century that were used by Ward-Perkins and the rest of the BSR team during the South Etruria survey – I’d love to get those done similar to this application, but I expect I’d run afoul of one or several intellectual property issues…)
BibleMap.org – An interactive map of locations from the Bible.
World Heritage Google Map – A Google Map of UNESCO World Heritage Sites worldwide that includes photos.
PlaceOpedia – A map of Wikipedia articles linked to their locations.
World’s Creepiest Places – Just as the name implies, this map shows information about the world’ creepiest places.
The Kremer Collection – Use a Google Maps interface to browse a large collection of paintings.
zkimmer – An online publication viewer for newspapers and magazines that uses the Google Maps UI.
Google Maps Recent Edits – A constantly updating map that shows the most recent edits to Google Maps.
We Tell Stories – A map of the stories of six different authors that lets you follow the trail of their stories around the world
Map Builder – A quick and easy Google Maps mashup builder.
MapMyPage – A simple tool to put Google Maps on your website.
Map My Life – An easy to use mashup that will map your life and show a timeline using an XML file you provide.
The Google Maps Image Cutter – A free application for cutting any image into tiles for use with the Google Maps interface.
Automatic Tile Cutter – Another app for cutting any image into tiles to use with Google Maps.
GMapCreator – A tool to make creating thematic Google Maps easier.
Geo Twitter – GeoCode your tweets and plot them on an embeddable map.
Mapmsg.com – An app that lets you put a message (smoke signals, crop circles, etc.) into a map and then email it to anyone.
Dual Maps – Free mapping tools to combine different Google Maps views as well as Microsoft Virtual Earth maps.
maps-for-free.com – Get relief layers for Google Maps free for creating your own mashups.
HeatMapAPI – Use this API to create your own heat maps to overlay Google Maps.
PdMarker – An app to help you easily customize Google Maps marker behavior
The original list lives here