I'm Bill Chadwick, a Google Maps API developer from the UK. In my spare time I like to go caving. We cavers like to see how caves relate to the surface features so overlaying cave surveys on Google Maps was something I wanted to try out. A cave map is just one kind of special map though - other kinds include footpath networks, cycling routes, public transport routes, flood risk areas, historic maps, etc.
Some of these special maps could be shown on Google Maps using KML files or GPolylines/GPolygons, but those techniques require that you have detailed coordinates of the objects you want to plot. What do you do if all you have is a map image (perhaps scanned in from paper)?
Well, Microsoft might just have your answer in the form of their free MapCruncher for Virtual Earth. MapCruncher will read in your map (jpg, transparent png, PDF, etc.), let you register it with the real world and at the click of a button, make a tile set ready for use as a custom GMaps tile layer. This works because by happy coincidence, Microsoft Virtual Earth and the Google Maps API use the same map projection and tiling scheme. MapCruncher takes care of all the rotating and warping of your map that is required for it to correctly appear in GMaps. An example of what can easily be achieved from a scanned-in map is shown in the map below. When the Satellite+Caves map is toggled on, you can drag the button on the +/- slider to adjust the transparency of the custom cave layer.
If this sounds like something you want to try with some maps of your own, you can read my comprehensive set of instructions on using MapCruncher with Google Maps. Oh, and in case you're wondering — the reason I am muddy and smiling in my portrait is because it was taken in late 2006 when we had just found this new cave passage.