Iconography

Bowed Strings Iconography Project, huge database of iconography featuring bowed strings with advanced search functions

Cello / Bass Violin Iconography Database, including statistical analysis of overhand/underhand bowings and instrument features by Daniel Rosin

Portraits of Unknown Baroque & Classical Era Musicians Facebook Group, thousands of images organized into convenient albums according to what you're interested in

Scores, Manuscripts, and Treatises

IMSLP Petrucci Music Library, a well-known resource for public domain scores, treatises, and recordings

BaroqueMusic.It, a collection of 17th and 18th century music in modern notation—although not as ideal as original manuscripts, it contains some works not published on IMSLP

RISM, a search engine for what musical sources exist and where they can be found

Early Music Online, a collection of manuscripts held by the British Library

The Düben Collection Database Catalogue, a digitized catalogue of the works contained in the Düben Collection

Many other scans of original manuscripts can be found through other institutions via a simple Google search.

Websites for Texts of Specific Sources

Leopold Mozart, Versuch einer gründlichen Violinschule, digital critical edition by the Stiftung Mozarteum Salzburg

Dictionaries of l’Accademia della Crusca, a Florentine dictionary published from the 17th through 20th century

Using Primary Sources

Transkribus, an AI-powered resource that transcribes scanned source material, whether handwritten or printed, into text. Use it with caution and a careful eye as it can make many mistakes!

Partifi, a tool for creating parts out of manuscript scores

Journal Articles and Books

JSTOR, a digital archive of scholarly articles, books, and images in many languages. Institutional access necessary unless you are rich and can afford to buy everything you need. 

Google Books and Google Scholar, search engines for books and scholarly research that offer public domain sources as well as the ability to read a few pages of certain books if you find a relevant chapter but don't have access to the whole book.

Bibliographie des Musikschrifttums online, another search engine for scholarly research, especially sources written in German. 

Definitions, Basic Knowledge, Biographies, and Language Help

Grove, the online version of Oxford’s English-language music encyclopedia. Institutional access necessary. Though it does not include entries for more obscure composers, the articles it does have are well-researched and peer reviewed.

Österreichisches Musiklexikon online, German-language music lexicon focusing on Austria’s musical history

MGG Online, German-language encyclopedia of music

Lily Pond Music Glossary, for translations of common musical terms in several languages

Music Dictionary Online, an extensive music dictionary. The website is named after Arnold Dolmetsch (1858–1940), one of the first 20th century figures interested in early music. Other sections of the website include biographies for lesser-known composers and explanations of music theory such as the fundamentals of figured bass.

Frühneuhochdeutsches Wörterbuch, dictionary of German vocabulary used in the mid-14th century to the mid-17th century