Martin Knoller art links 

Liechtenstein Museum, Vienna
Nikolaus II, Prince Esterházy, 1793 (Zoom)

Louvre Museum Database, Paris

Art Gallery of Greater Victoria, British Columbia

 

Martin Knoller in the Web Gallery of Art  

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

Farbdia Archiv (Color Slide Archive) (in German)  
Thousands of photographs of fresco cycles and wall decorations in churches and other buildings in and around Germany.
Click "Galerie" for a normal overview; click "digilib" to zoom in.

 

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.