Vienna, Austria - Here is a map for the Vienna Underground transit system. With the addition of street maps, this is a very functional mashup for the city of Vienna, Austria. More information on the map is in this blog post.
Germany - Here is a zip code viewer for those of you in Germany. The tool is completely in German and lets you type in German zip codes (plz) and the borders are then displayed on the Google Map. This tool has quickly been retrofitted with street maps as well.
Brussels, Belgium - Find a cafe or bar and browse locations of wall paintings in this mashup for Brussels. This mashup includes the street maps viewing option.
Paris, France - Here is a map of kosher restaurants in Paris. Restaurants are highlighted as pins on the map and upon clicking you can see all the metro stations that are close by. This mashup does need a quick retrofit to offer the user the option to toggle street maps on.
UK Bed & Breakfast Map
Gothic Architecture in France (click "G" in the Maps column)