Two websites offer some wonderful comparisons of 19th-century photographs of Paris with contemporary views.
This site at USC has rephotographed locations originally shot by legendary Paris photographer Charles Marville between 1859 and 1877. Some of Marville's photos are before Haussmann's reconstruction of Paris; others are after.
This site was compiled by the French newspaper Le Figaro, so the captions are in French, but they are not too complicated to figure out if you look for the place names. They give you a good feeling for the way the city changed during the Second Empire.