Jan van den Hoecke art links 

Kunsthistorisches Museum Databank, Vienna (in German)  

The Royal Collection, London, UK
(Zoomable - look for the magnifying glass and the text "Magnify image")

Kimbell Art Museum, Fort Worth, Texas
The Triumph of David

State Museums of Florence Digital Archive, Italy

Virtual Uffizi

 
National Art Databases and Museum Inventories:

Flemish Art Collection Database, Belgium (in Dutch)  

 

Wikimedia Commons Image Database  

World Visit Guide (formerly Insecula)

Bildindex der Kunst und Architektur (in German)  
(a welcome screen is displayed for a couple of seconds before the search results appear)

Ciudad de la Pintura (in Spanish)  

 

Artarchiv Sample Artist Symbols + Monograms
Scroll down to monogram 465

UK Government Art Collection

 

Grove Dictionary of Art Online (excerpt)  

Union List of Artist Names (Getty Museum)  
Reference sheet with basic information about the artist and pointers to other references.