For some years I have been using the French online mapping portal www.geoportail.fr
As a French government site it is full of Map related information and cartography. Principally there are the online IGN maps all with adjustable scales. Not only does it cover mainland France but also all the overseas territoires.
A new function available are on line SHOM Charts. The Menu on the left hand side of the window offers a range of selectable features, by default the window opens in Aerial photo view, to view Maps select CARTES then on the Sub menu Cartes IGN. Each selection adds a new mapping layer to the window
At the bottom of the menu it is possible to adjust the opacity of different layers with a slider.
Thanks to Michelle for discovering this new addition