I've probably linked to this before but it's worth linking to again. And again. 520,000 images. That should keep you amused for a couple of evenings.

NYPL Digital Gallery

NYPL Digital Gallery provides access to over 520,000 images digitized from primary sources and printed rarities in the collections of The New York Public Library, including illuminated manuscripts, historical maps, vintage posters, rare prints and photographs, illustrated books, printed ephemera, and more.

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  thanks to daily KOS

Here are a couple of sections.

Ellis Island Photographs from the Collection of William Williams, Commissioner of Immigration, 1902-1913

Lewis Wickes Hine: Documentary Photographs, 1905-1938