Music Research and Resources

Description from Wikipedia: The International Music Score Library Project, also known as the Petrucci Music Library after publisher Ottaviano Petrucci, is a subscription-based project for the creation of a virtual library of public-domain music scores.

An index and searchable database of free Leider and Art Song sheet music for singers and educators.

From the website: The Répertoire International des Sources Musicales (RISM) is an international, non-profit organization with the aim of comprehensively documenting extant musical sources anywhere in the world.
Cataloging musical sources is financed and carried out by various national and international institutions.
Independent national working groups at libraries and archives in many countries worldwide catalog historical musical sources: music prints, music manuscripts, libretti, and theoretical writings about music. The results are edited and published by RISM.