The Parish Book of Chant - downloadable book of Gregorian chant neumes

This rocks (well, quite the opposite): The Parish Book of Chant, available in its entirety for download. It includes the Pater Noster and Magnificat, which is great if you want to start adding some chant to Evening Prayer.

MusicaSacra has also scanned the Liber Usualis, a 2340-page book of common Gregorian chants (115 MB PDF).