Tim Smith, a music theorist and choral conductor at Northern Arizona University, has created a great website on Bach's two books of keyboard works exploring the limits of tuning and keys. This website combines scores with digital recordings of Smith's colleague, David Korevaar, and offers analyses of both theoretical and historical interest.
As someone who will be teaching a counterpoint class next spring, I am very excited about the Index of Contrapuntal Operations, Learning Objects, and Concepts. This lists the number of voices in each prelude and fugue, the number of subject entries, whether the answer is tonal or real, if there is a countersubject, and whether the piece has an instance of specific types of contrapuntal operations: stretto, melodic inversion, invertible counterpoint, augmentation, and the BACH motive (not really a contrapuntal operation, but still very useful). Beneath the index is a description of key concepts that
are involved in each prelude or fugue, along with a list of questions that students can answer online and email to instructors. These questions require the student to read the provided narratives, check out links like Bach Digital for sketch study, listen to and analyze the specific piece, and relate the specific piece to the overall collection. This is an excellent source for teachers, and one that I plan to use extensively.
(via Crooked Timber)