Of the many wonderful online resources, the following are some of my favorites.

General Photo Reference:

• Luminous Lint (Alan Griffiths's labor of love and a fantastic resource)

• Robert Leggat's History of Photography

• The Victoria and Albert Museum's Photographic Processes page

• The Met's Photographic Department

• The incomparable George Eastman Museum has access to their online collections here

• The International Center of Photography

• Craig's Daguerreian Registry

• The Daguerreian Society

• The Impossible (Polaroid) Project

• Photobooths

• Photogravures

• David Cycleback on Judging the Authenticity of Photographs

Found/Vernacular/Snapshot Photography Reference:

• Nick Osborne's collection (for my money, the greatest collection of vernacular photography currently online; the product of an exquisite eye) is documented on his facebook page.

• John Foster's wonderful Accidental Mysteries

• Joel Rotenberg's excellent article on the art of the snapshot

• A teriffic interview with Robert Jackson about "pure [snapshot] photography" and Robert Jackson's superb instagram account.

 

And some facebook groups which may be of interest:

• Victorian Images Classified

• Snapshot Mafia

• Available Snapshots

...and there are many, many more