Cantus (U. of Western Ontario, Canada)
http://publish.uwo.ca/~cantus/
[A database for Gregorian Chant. Indices of chants in selected manuscripts and early printed
sources of the Divine Office.]
Gregorian Chant Home
Page (Princeton U., USA)
http://www.music.princeton.edu:80/chant_html/
[Excellent collection of links to resource sites, historical information.]
The Gregorian Association (London, England)
http://www.beaufort.demon.co.uk/chant.htm
[Historical background, resources, and links.]
Gregorian Chant Tutorial
gopher://osiris.wu-wien.ac.at:7121/11/pub/earlym-l/gregorian.chants
[A plain text version of a short tutorial in four parts by Donald J. Casadonte of Ohio State University.]
Gregorianischer Choral
http://www.aiscgre.de/
[Resources and links in German from the International Society for Study of Gregorian Chant.]
Harmonia
http://www.indiana.edu/~harmonia/
[Early music on public radio in North America.]
Plainsong and Mediaeval Music Society
http://www.ncl.ac.uk/~nip2/
[Not currently online.]
The Saint Gregory Society
http://www.saint-gregory.org/
[Background and resources and items for sale from the Society, located in New Haven, Connecticut, USA.]
Sets of Resource Links
Gregorian Chant Web Guide
http://www.msu.edu/~knitter/chantlinks.html
[A very well selected and maintained page of chant links that are collected and annotated by Scott Robert Knitter.]