Sites of free access.
1)
http://signaturbogen.wikidot.com/start - signature and monogram of Danish artists.
2)
http://www.artsignaturedictionary.com/- signature and monogram Western European (mainly Scandinavian) artists. There is a search by name and monogram. There is advanced search, where you can specify only a partial name or generally known to the letter, replacing the unknown marked by an *. Registration - free. There is a base and forged signatures (but for a fee).
3)
http://www.auktion-bergmann.de/Katal..._id=3&s_id=300 - signature and monogram Western (mostly German and Austrian) artists.