"The ultimate goal of the IMSLP is to gather all public domain music scores, in addition to the music scores of all contemporary composers (or their estates) who wish to release them to the public free of charge."
Nearly all of the major labels are represented, and new releases tend to get added promptly. Also includes a certain amount of musical theater. Create a personal login; then download one of their mobile apps to take music with you (you can download recordings for offline listening).
"Full text from major reference works. Personal narratives from oral histories and interviews. Manuscripts, songsheets, lyrics, discography data, and other text sources. The largest online collection of musical scores and reference works available anywhere."
"The database includes songs by and about American Indians, miners, immigrants, slaves, children, pioneers and cowboys. Included in the database are the songs of Civil Rights, political campaigns, Prohibition, the Revolutionary War, the Civil War, anti-war protests and more."
"Reggae, worldbeat, neo-traditional, world fusion, Balkanic jazz, African film, Bollywood, Arab swing and jazz, and other genres such as traditional music - Indian classical, fado, flamenco, klezmer, zydeco, gospel, gagaku, and more."
"Daniels' Orchestral Music Online is a unique orchestral music finder tool and an invaluable resource for orchestral librarians, libraries, planners, musicologists, conductors, and artistic directors. You can browse the classical repertoire from Karl Friedrich Abel to Ellen Taffee Zwilich and find basic information on composers and instrumentation plus inspiration for advanced programming. You can also search opera arias, duets choruses, and more using the opera excerpts finder."