General

VIAF - Virtual International Authority File

World Biographical Information System

WorldCat: The world's largest network of library content and services

Union List of Artist Names, compiled by the Getty Research Institute with systematic listings of name variants and brief biographical information; only lists well-known artists

Karlsruhe Virtueller Katalog with multiple search options in international library catalogues, online resources and commercial book providers

Global search option over rich holdings of the Smithsonian Institution in Washington, D.C., whose book catalogues are usually encompassing all names of illustrators, engravers, lithographers etc.

Same for the Gallica-catalogue of the Bibliothèque Nationale de France in Paris

Same for the Natural History Museum, London

Wellcome Institution Images: 2000 years of human cultures

New York Public Library; for global searching or more specifically for scientific images click here
Yale University's Digital Content, Global Search

Netherland's Rijksbureau voor Kunsthistorische Documentatie (Dutch: Rijksbureau voor Kunsthistorische Documentatie or RKD) with direct link to the databases of artists, images, and literature

ARTSTORE An Image Library for the Arts and Sciences (with more than 1 million images, mostly from the arts, architecture, humanities and social sciences)

wikipedia.org for illustrators already listed there, these often brief entries typically also include a few samples of his or her work and/or links to other internet sites featuring more detailed bios or analysis

https://es.wikipedia.org/wiki/Wikiproyecto:Mujeres/Art%C3%ADculos_faltantes/Ilustradoras   for female illustrators without wikipedia entry, only some of the scientific illustrators though

HeidICON - Heidelberg Bilddatenbank (with focus on the middle ages)

Bildindex der Kunst und Architektur Deutsches Dokumentationszentrum für Kunstgeschichte und Bildarchiv Foto Marburg

Oxford Index - A Search and Discovery Gateway across OUP's publications, including the latest Engl. ed. of -> Benezit (2006)

The British Museum - with an online index allowing name, place, subject, technique, schools/style and material searches

Photography & related techniques

Craig's Daguerreian Registry - Resource on American Daguerretypists & Photographers 1839-1860

Photographen wiki  https://www.fotografenwiki.org/index.php/FotografenWiki:Hauptseite 

http://www.luminous-lint.com/app/home/ photography: history, evolution and analysis

http://erps.dmu.ac.uk/exhibit_details.php?etid=143265  catalogue records of exhibitions of the Royal Photographic Society 1870-1915

Natural History Expeditions

On Scientific Voyages Around the World 1679-1879

Miguel Angel Puig Samper (2012): Illustrators of the Spanish Scientific Expeditions of the enlightenment

General Biology

BHL Biodiversity Heritage Library 

Science Gossip, with more than 120.000 classified illustrations from Victorian science journals

https://en.wikipedia.org/wiki/Biological_illustration

Botany

www.plantillustrations.org esp. their search list http://plantillustrations.org/artists.php?

wikipedia on botanical illustrators

List of Australian botanical illustrators

Early botany illustrated

Australian Plant Collectors and Illustrators 1780s-1980s Australian National Herbarium

"Women Illustrators". The Art of Botanical Illustration. University of Delaware Library 

Hunt Institute for Botanical Documentation Carnegie Mellon Univ., Pittsburgh (publications)

http://collections.mnh.si.edu/search/botany/?ti=6 allows a search for botanical illustrators & artists in the Collections of the Smithsonian Institution in Washington, D.C.

Herbals from the Middle Ages and Early Modern Period: specifically on carrots in dozens of mss. and links for fully scanned version of 10 works from 1475 to 1537.

Botanical Illustrations de description de l'Egypte:  http://gallica.bnf.fr/ark:/12148/btv1b23003011

Botany in British India 1800-1850: https://www.bl.uk/collection-guides/botany-in-british-india#

Zoology

Museum Victoria on zoological illustrations from Colonial Victoria (Australia)

http://www.gutenberg.org/files/39471/39471-h/39471-h.htm William Swainson: Zoological Illustration, London 1820-21

https://museumvictoria.com.au/caughtandcoloured/ Zoological Illustrations from Colonial Victoria

https://commons.wikimedia.org/wiki/Category:Zoological_illustrations

http://www3.amherst.edu/~thoughts/contents/peterson-ichthyology-appendix.html

Ornithology

global search through various specialized bird journals

Crystallography & Mineralogy

The Mineralogical Record (with excellent search options for names & collections)

Virtual Museum of the History of Mineralogy Early Crystal Models 

Mineralogical Illustrations de description de l'Egypte:  http://gallica.bnf.fr/ark:/12148/btv1b23003011 

Des cristaux et des hommes: la minéralogie au muséum

Geography & Geology, esp. Maps

www.geographicus.com and http://www.geographicus-archive.com/ for zoomable maps

http://portal.digmap.eu/index/

https://www.loc.gov/maps/?st=slideshow

https://www.oldmapsonline.org/

http://www.birmingham.ac.uk/facilities/lapworth-museum/collections/maps/index.aspx
http://special.lib.gla.ac.uk/exhibns/maps/maps_19th_century.html

http://www.davidrumsey.com/ a large collection of c.36000 maps and images

http://www.lindahall.org/events_exhib/exhibit/exhibits/vulcan/ exhibition of 66 rare books and journals, published between 1565 and 1835 at the Linda Hall Library

http://www.johnnurmisensaatio.fi/?cat=59 further antique maps with emphasis on Scandinavia

General Medicine

http://www.ncbi.nlm.nih.gov/pubmed comprising more than 22 million citations for biomedical literature

images from the history of medicine

MEDLINE (Medical Literature Analysis and Retrieval System Online) and SEARCHING: UltraMED [Including MEDLINE] 1966-2012 

http://www.biusante.parisdescartes.fr/histoire/medica/ searchable for authors & various specialties, e.g.,

http://www.biusante.parisdescartes.fr/histoire/medica/resultats/?intro=neurosciences&statut=charge

https://www.ub.ruhr-uni-bochum.de/DigiBib/Aktuelles/BildermitLeg.pdf  Irmgard Müller: Ausgewählte Abbildungen aus Werken der ehemaligen Deutschen Ärztebibliothek

Anatomy

http://link.library.utoronto.ca/anatomia/application/index.cfm c.4500 anatomical plates from the rare book library of the University of Toronto 

http://www.designboom.com/history/anatomical_maps.html 

http://www.nlm.nih.gov/dreamanatomy/ Dream Anatomy: A National Library of Medicine Exhibit

Pop-up Anatomy: https://www.pinterest.com/olilag/pop-up-anatomy/

Physiology and Histology

CSlide - Physiological and histological lithographs and chromolithographs in late 19c Cambridge

Astronomy 

The Birth of Selenography: http://brunelleschi.imss.fi.it/galileopalazzostrozzi/section/TheBirthOfSelenography.html

Felice Stoppa: Atlas coelestis (w. many links to online astronomical plates from 1320 to 1912)  http://www.atlascoelestis.com/acquista.htm

Chemistry & Alchemy

The Alchemy website on levity.com with more than 2500 alchemical drawings & paintings

Mathematics

Mathematical Treasures: The Linda Hall Library

Technology and Architecture (esp. civil engineering)

Centuries of Civil Engineering A Rare Book Exhibition Celebrating the Heritage of Civil Engineering 

"Architektur- und Ingenieurzeichnungen" more than 100.000 Architectural Drawings and Engineering Drawings from the Collections of the Saxonian Library (Sammlungen der SLUB), the architecture museums of the TU Berlin  (Architekturmuseum der Technischen Universität Berlin) and TU Munich (Architekturmuseum der Technischen Universität München) also including the DFG-project Architektur- und Ingenieurzeichnungen der deutschen Renaissance. Digitalisierung und wissenschaftliche Erschließung des Zeichnungsbestandes von 1500 – 1650

Pedagogy and Didactics

pictura paedagogica online an image archive for the history of teaching (Bildarchiv zur Bildungsgeschichte), allows searches for people, journals, books, and keywords.