I found it on the website of the Paris Musées, a group of 14 museums in Paris that have now made roughly 100,000 images from across their collections available for free download.
As with all online collections like this, random browsing is time-consuming, but if you know exactly what you're searching for, it's a potential gold mine. For instance, if you're looking for images of what Parisian architecture looked like in the 19th and early 20th Centuries, there's tons of the stuff.
Lansiaux, Charles Joseph Antoine. 33, rue Saint-Honoré, 1st arrondissement, Paris.
Atget, Eugène. Passage Beaujolais, 47 rue Montpensier, 1st arrondissement, Paris.
Lansiaux, Charles Joseph Antoine. Rue Jean-Robert (9 and 11-3-1918).
Atget, Eugène. Corner of Place Saint-André-des-Arts and Rue Hautefeuille, 6th arrondissement, Paris.
Atget, Eugène. Place Saint-André-des-Arts, 6th arrondissement, Paris.