Presser Music Library

Library Score Databases

Free Online Scores

IMSLP: Petrucci Music Library.  Over 700,000 scores in the public domain.

Choral Public Domain Library.   Over 46,000 choral and vocal works by over 4,700 composers. 

Mutopia Project. 2124 pieces of music – free to download, modify, print, copy, distribute, perform, and record – all in the Public Domain or under Creative Commons licenses, in PDF, MIDI, and editable LilyPond file formats.

Scores on Demand. Perusal scores from members of Wise Music Group: Chester Music, Novello & Co, G. Schirmer/AMP, Edition Wilhelm Hansen; Unión Musical.

List of Online Digital Musical Document Libraries. From Wikipedia, a list of more than 50 websites with digital scores.