This sites includes most of Mozart's scores for both his sacred and secular works. Some of the scores include audio options. Some of the more useful information (such as critiques) are, unfortunately, in German.
This Bach site offers English translations to the vocal works of J. S. Bach along with introductory information on the sources and on performance history of each work. Commentary on the text is found in footnotes (opened by clicking on the footnote number). The present translation is offered to listeners, interpreters, and performers as a guide to the vocal works of J.S. Bach. Virtually the complete corpus is included. It includes the texts of lost compositions, but does not include untexted chorales.
The Petrucci Online Library of Musical Scores has over 27,000 scores in a variety of genres including Instrumental (chamber and other), Opera, Vocal (non-operatic) and books. All scores are in the public domain. Works are Categorized by genre or you can search directly by composer name in the search box and clicking the "Comp." button. Books include works on the history of music, biographies and critics, pedgogical works (theory and instruments) and libretti. You will see work in languages other than English.
However, another main goal of IMSLP is to facilitate the exchange of musical ideas outside of compositions: for example, the analysis of a particular piece of music. Therefore, feel free to create/edit a page with your analysis of a particular piece (please use the "Discussion" link on the work page of that particular piece). For general discussions, and IMSLP-related questions, score requests, etc. you can use the forums. We hope to build a growing community of dedicated musicians and music lovers, who can use this site as a platform for enjoying music.
IMSLP was started in 2006. The logo on the main page is a capital letter A. It was taken from the beginning of the very first printed book of music, the Harmonice Musices Odhecaton. It was published in Venice in 1501 by Ottaviano Petrucci, the library's namesake.
