General references

(References specific to particular publishers will be found via the appropriate logo).

Postcard logos French
The privately published book "Dictionnaire de la Cartophilie francophone", Paris, P. Armand, 1990 looks as though it would answer many questions particularly on regional publishers, if you are lucky enough to have access to it.
Some Italian publishers
misterkappa, Italian publishers, good site gone or moved but see on the wayback machibe.
www.calcografica.it. This doesn't cover postcards but is an amazing site for prints and photographs.
Postcard logos, Belgian based but general These sites are far from complete, but do cover a the full range of publishers - ie they include the many publishers of topographic (view) cards which are not covered in this site and also more modern cards. The Belgian site, sponsored by Delcampe, is worth the trouble of registering for (it took me several gos) as you can then see many example cards. The French site is also worth registering for.
Russian postcards. I am not competent to include Russian cards here, but this is a nice article with good images.
TPA, The Postcard Album A key English language reference on German postcard publishers and printers. Includes detailed articles, with particular interest in business and technical sides of production. Also produces a magazine.
German postcard logos A subset of the TPA site.
Dutch cards I have only just discovered this site but it looks to be a treasurehouse of information on Dutch cards particularly.

1,5 millions de cartes postales par jour en 1914. Qui étaient les éditeurs Article and refs on French Patriotic postcards 1914--1918.
Propaganda Postcards of the Great War: the site I gave for this has disappeared or moved, but a web search-engine will show various collections.
World War 1 postcards from the Bowman Gray Collection - about 10,000 cards in hi-res.
metropostcard.com: General reference.
*** The version of this site referred to here is now "temporarily available" at web.archive.org/web/20220330075923/http://www.metropostcard.com/.
History of Prague postcards. Details of domestic and foreign publishers active in the country.
Vintage Japanese postcards. Not really relevant to this particular collection but interesting.
PUBLISHERS' TRADEMARKS IDENTIFIED , compiled in 1962 by Walter E. Corson, edited and enlarged by James Lewis Lowe in 1993, and published by Deltiologists of America, Norwood, PA; 112 pages, 8.5" x 11" softback, illustrations of 1562 trademarks and styles of printing 'Post Card' on card backs. Corson created this unique book using scissors, Lowe added more organised material. Only available secondhand.

Wikipedia Commons now has a very good and easily searchable section on postcard logos which covers postcards of all kinds.

Useful information on dating postcards is here.

Refs I should have included earlier:

Dictionnaire des imprimeurs-lithographes du XIXe siècle (French ones) 19th century litho printers.

The Waddleton Chronology - Search the colour printers database.

undividedbackpostcard.com. This site has lots of information and examples of early cards, not limited by subject or origin.

Raphael Kirchner: life, works, postcards.

Directory of Belgian Photographers.

STAMPS. There is no shortage of information about postage stamps on the web, my current favouite site is worldstampsproject.org/.

Stamp boxes if you can see them can provide information. See Real Photo Postcard Stamp Boxes and Horseshoe & T.I.C Stamp Square. These stamp box references are mainly useful for collectors of real photo cards produced by smaller firms.