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http://www.harbrace.com/art/gardner/RenBar.html#Italy - Gateway to Art History, somewhat fragmentary, can be annoying at times.
http://mirrors.telepac.pt/CJFA/altindex3.html#it14 - Carol Gerten's Fine Art - EXCELLENT! A comprehensive listing of artworks by artist. I spent hours going through this site.
http://www.ocaiw.com/pintura.htm - Orazio Centura's Art Images on the Web
http://www.tigtail.org/TVM/X1/early.html - TigerTail's Gallery of the Early Italian Renaissance (mixed art forms, sculpture, not comprehensive)
http://www.arca.net/uffizi/index1.htm - The Uffizi Gallery - Very Good - lotsa religious pictures with excellent information on the provenance of the pictures
http://sunserv.kfki.hu/~arthp/artist4.html#t - takes some searching, but good