Ars Magica

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/

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

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

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/

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

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

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