While OpenStreetMap is rather good for cartographic information (and rendered maps) in urban areas of France, it is rather unsatisfactory in rural areas (except in the area where I live, where I improved the map!) The IGN local maps (1 : 25,000) are the reference maps for these, and include a high level of cartographic details while covering all of France. They are available online from the government-subsidized Géoportail, but are not part of the data made available through the French government open data effort (list of data available detailed here in French). In particular, while there exist an API to query information, it cannot be downloaded for offline use (as far as I am aware).
Where can I download high-precision cartographic data of France, with a good coverage of rural areas?