GENERAL

For images of Rembrandt's works generally:
http://www.pubhist.com/person/1/rembrandt/works with a separate section on Rembrandt's drawings:
http://www.pubhist.com/person/1/rembrandt/works/drawings

DOCUMENTS:
The Rembrandt Document Project (RemDoc) is a joint initiative of the Radboud University Nijmegen, the Rembrandt House Museum (Museum het Rembrandthuis) in Amsterdam, and the Huygens Institute for the History of the Netherlands. RemDoc aims to include all the known documents that relate to the life and work of Rembrandt produced from the 15th to the 18th century:
http://remdoc.huygens.knaw.nl/#/document/remdoc/e14075

PAINTINGS
For images of Rembrandt's paintings:
http://staff.science.uva.nl/~fjseins/RembrandtCatalogue/r_1624_1631.html
Also, growing in this area, with a focus on documentation concerning technical examinations, see:
http://www.rembrandtdatabase.org/Rembrandt/
This site has published the first 5 volumes of A Corpus of Rembrandt Paintings (1982-2011) in PDF form online at:
http://www.rembrandtdatabase.org/Rembrandt/cms/corpus

MAJOR DRAWINGS COLLECTIONS
AMSTERDAM, RIJKSMUSEUM:
https://www.rijksmuseum.nl/en/search?q=rembrandt&p=1&ps=12&maker=Rembrandt+Harmensz.+van+Rijn&role=tekenaar

LONDON, BRITISH MUSEUM: http://www.britishmuseum.org/research/publications/online_research_catalogues/rembrandt_drawings/drawings_by_rembrandt.aspx
This is the catalogue listed in the bibliography here as London, 2010 (online).
The catalogue is also available in PDF format (5 files) for those who would like to download or even print it:
Catalogue of Drawings by Rembrandt and his School in the British Museum.

VOL. I. Introductory Material, catalogue list, bibliography, Drawings by or Retouched by Rembrandt Cat. 1-72 19.0MB
VOL. II. Attributed to Rembrandt and Copies after Rembrandt Cat.73-97. Anon. School of Rembrandt Cat .98-136 12.5MB
VOL. III. Named artists of the Rembrandt School: Backer - Borssom 15.0MB
VOL. IV. Named artists of the Rembrandt School. Dijk - Furnerius 14.5MB
VOL. V. Named artists of the Rembrandt School. Gelder - De With 19.0MB

DOWNLOAD PDF FROM LINK BARS:

19.0 MB

12.5 MB

15.0 MB

14.5 MB

19.0 MB

PARIS, MUSEE DU LOUVRE:
http://arts-graphiques.louvre.fr/fo/visite?srv=ra

VIENNA, ALBERTINA:
http://sammlungenonline.albertina.at/default.aspx?lng=english

COLLECTORS' MARKS:
For collectors' marks found on works on paper, see the database founded on the books published by Frits Lugt, 1921 and 1956 (see Bibliography):
http://www.marquesdecollections.fr/

BIBLIOGRAPHY
The Bibliography of the History of Art and RILA (via the Getty Research Institute):
http://library.getty.edu:7108/vwebv/searchBasic

KUBIKAT - online list of art-historical publications produced collaboratively by the Kunsthistorisches Institut in Florenz (Florence), the Zentralinstitut für Kunstgeschichte (Munich), the Deutsches Forum für Kunstgeschichte (Paris) and the Bibliotheca Hertziana (Rome) at:
http://aleph.mpg.de/F?func=file&file_name=find-b&local_base=kub01&con_lng=eng 

ICONOGRAPHY
The Warburg Institute:
http://warburg.sas.ac.uk/vpc/VPC_search/main_page.php

OTHER
For a list of links to sites concerned with Dutch and Flemish art, see:
http://www.hnanews.org/hna/links.html