Sources (manuscripts etc.)

RISM (Répertoire International des Sources Musicales)
About 700,000 entries for research according to music source (especially historical, handwritten music written before 1800).

Richard-Strauss-Quellenverzeichnis
Music sources from the first outline to the first edition of every single piece by Richard Strauss.

Schubert-Autographe
Contains digital reproductions of more than 500 music autographs, letters and biographical documents by Franz Schubert.
Currently, the music manuscripts of the music collection of Wienbibliothek im Rathaus, Vienna City Library, can be viewed here.

Instrumental music of ther Dresdner Hofkapelle orchestra
The digital collection of the Saxon State and University Library Dresden mainly contains works of the Dresdner Hofkapelle from the first half of the 18th century.
In total, about 1750 manuscripts.

Düben collection Uppsala
Approx. 2,300 pieces from the 17th and early 18th century in manuscript form (including those by Buxtehude, Byrd, Caldara, Frescobaldi, Schmelzer).