Francesco de Mura, Allegory of the Arts, ca. 1750
Gallica: the electronic portal of the Bibliothèque nationale de France, with an immense and growing number of digitized books and scores, including many original editions and manuscripts of eighteenth-century operas http://gallica.bnf.fr/?lang=EN
Getty Research Institute: rich collections on 17th- and 18th-century European spectacles, including such subcollections as the Italian Theater Collection (books and manuscripts) Italian Theater Prints (iconography) and the Festival Collection http://www.getty.edu/research
IMSLP/Petrucci: the International Music Score Library Project; a repository of (mostly) out-of-copyright digitized scores, increasingly including original editions and even composers’ autographs (imported from other sites) http://imslp.org/wiki/Main_Page
Metropolitan Museum, Heilbrunn Timeline of Art History, 1600–1800: annotated slide shows of artworks and musical instruments from the collection http://www.metmuseum.org/toah/world-regions/#/09/Europe
Die Oper in Italien und Deutschland zwischen 1770 und 1830: digitized manuscript scores and other information on operas from this period (German only) http://www.oper-um-1800.uni-koeln.de/
Pietro Metastasio: Drammi per musica – full electronic texts of the librettist’s drammi per musica, and other useful links www.progettometastasio.it/pietrometastasio
Society for Eighteenth-Century Music (SECM): links, newsletter, online texts (just one, so far) http://www.secm.org
French Revolution Digital Archive: https://frda.stanford.edu/