From Nosolosig (a Spanish meeting point of the community interested in Geographic Information Technologies)
Multimapas, an interesting tool to compare maps.
"Javier Jimenez Shaw has developed and maintains
Multimapas to compare maps from different periods, using the Google Maps API."
"From the web page we can choose between different maps and choose how we want to see them: superimposed, with transparency; With windows synchronized side by side, or continuously, showing different base maps."