On March 21, 2016, we celebrate Johann Sebastian Bach's 331st birthday. Here are links to freely available resources, including some courtesy of Stanford University.
Stanford University Libraries holds one original Bach manuscript, the basso part from the cantata "Ich liebe den Höchsten von ganzem Gemüte Greifet zu, fasst das Heil", BWV 174. A high-quality digital image is available for viewing and download.
The Neue Ausgabe sämtlicher Werke (new edition of the complete works) is shelved in the Music Library stacks.
The 19th-century Bach Gesellschaft edition of the complete works is available for download via the Petrucci Music Library (IMSLP)
The Bach Digital database provides links to most of the known manuscripts in existence and tracks Bach scholarship worldwide.