TopPlus-Web-Open from the German Federal Agency for Cartography and Geodesy (Bundesamt für Kartographie und Geodäsie or BKG) is "a consistent web map, completely based on free or open data sources.... The web map is available as Internet service via standardized interfaces (WMS/WMTS) and can be used freely by anyone." Although note that only Germany and neighbouring countries are available at large scale.
BKG also offer other
open and free maps and data.