Vermeer's signature, The Astronomer, 1668.
Musée du Louve, Paris. Source: Essential Vermeer.
Antique Jewelry University
The AJU includes listing of jewellery maker's marks.
Art Signature Dictionary
Signatures of over 40,000 artists, as well as auction results for the signed works and a brief biography.
Benezit Dictionary of Artists
Some entries include signatures.
ICD (International Ceramics Directory)
Links to examples of ceramic marks by country of origin.
Silver Marks, Hallmarks & Maker's Marks
Over 12,000 examples.