John Coryat created an example of a Google-like mashup using the OpenLayers system that uses a custom tile layer and WGS84 coordinates to mimic the behavior of a Google Maps API page.
OpenLayers is a relatively new API that uses open source Javascript libraries to display objects in a browser, similar to the Google or Yahoo systems but without the limitations imposed by those providers regarding terms of use. OpenLayers does not provide maps themselves or geocoders like the other providers but they do provide a robust library of functions that are well behaved.
See the example at:
http://www.usnaviguide.com/openlayers/examples/usnaviguide.htm