This project of the Internationale Stiftung Mozarteum and The Packard Humanities Institute provides access to digital editions of Mozart's works for personal study and educational use. PDF files of the works are available under the "Table of Contents" link.
Hosted by John Hopkins University, "The Lester S. Levy Collection of Sheet Music consists of over 29,000 pieces of American popular music. The collection spans the years 1780 to 1980, but its strength is its throrough documentation of nineteenth-century America through popular music".
A semi-definitive wiki of Christian hymns. Contents are organized by Hymns, Composers, Authors and Hymnals. Includes more than 780+ Hymn lyrics & stories, 300+ Authors biographies, details on 100+ Hymnals and some descriptions of Composers.
A comprehensive index of 5,160 hymnals and 1,096,884 published hymns, with scores, media, and information from sources such as Julian's Dictionary of Hymnology, hymnal handbooks, and experts in the field.