MusicaSacra - The website of the Church Music Association of America.  Includes several resources such as e-publications, forums, audio, and CMAA membership information.  Also of particular interest are the archives of Caecilia, Sacred Music, and Gregorian Review.

ChantDiscography.com

Corpus Christi Watershed

New Liturgical Movement

Cantus - indices of the Latin ecclesiastical chants found in early manuscript and printed sources for the liturgical Office, such as antiphoners and breviaries.

Indices

Analecta hymnica medii aevi - Contains authoritative texts of hundreds of medieval hymns.

The Hymn Tune Index 

Hymnary - a comprehensive index of hymns and hymnals.

Music

Choral Public Domain Library - Over 2800 scores available for free in PDF format. Lists other free sites.

Cyber Hymnal - Lyrics, scores, MIDI files, pictures, history, and other information on over 2,300 Christian hymns and gospel songs from many denominations.

Introits of the Sarum Rite

Liber Usualis Online

The Plainchant Gradual Vols 1 and 2
The Plainchant Gradual Vols 3 and 4

The Salisbury Antiphoner