Ars Magica

Hermes' Portal - A fanzine about the game.
http://pagesperso-orange.fr/styren/hermesportal/hermes.htm

Rabenstein Covenant - Offers characters, maps of the covenant, house rules, summary of the saga, and historical background.
http://www.unc.edu/~murphy/rabenstein/index2.html

Wikipedia - Contains history, setting, system information, and references.
http://en.wikipedia.org/wiki/Ars_Magica

Secretum Secretorum - Background for the game, contains information from a text translated from Arabic in the middle of the twelfth century.
http://www.granta.demon.co.uk/arsm/jg/index.html

Atlas games - Ars Magica - Publisher's site. "Come to a land of adventure and mystery, of falling empires and rising nations, of legendary creatures and incredible myths. This is Ars Magica's Mythic Europe, where the power of magic is real and exists alongside historical figures and locations. Stories revolve around the covenant: one part mystical college, one part magical laboratory, one part Hermetic fortification, one part bastion of secrecy."
http://www.atlas-games.com/arsmagica/

Project Redcap - The Crossroads of the Order - Features an Ars Magica FAQ, mailing lists, resources, and a large link collection for multiple languages.
http://redcap.org/

Cliffheart Covenant - A nicely laid out site about Ars Magica that was originally done in Italian, but has now been translated to English.
http://www.arsmagica.it/Cliffheart/index-en.htm