Ars Magica is a fantasy rules system and campaign setting which emphasis magic and wizards above everything else. There is no attempt to balance Magi characters with mundanes - magi are simply more powerful and generally flexible in all but a few situations. It is published by Atlas Games.
The game backs this up with a detailed and comprehensive magic system, which is second to none. The flexibility and detail available to players when designing and player magi characters is extensive, and makes Ars Magica one of the best game systems available. The game is focussed on the overall campaign spanning decades if not centuries - magi learn, adventure, build up their powerbase and eventually grow old and die over the course of it.
Even for those who have no intention of actually playing the game, it is worth getting just to mine it for ideas.
Mythic Europe : Druids' Dale
Mythic Europe : The Midnight Saga
Based on ideas from the Midnight D&D campaign setting, this saga is set in a Europe that is nearing the End Times when the forces of darkness will dominate the world of Mythic Europe. Needless to say, a lot of what will happen will break from 'canon' Ars Magica. Recent events include Heaven becoming apparently closed to new souls, the fading of the Divine from the world and the dead refusing to stay dead. Several magi who have died seem to have retained both their cognitive and magical abilities after awakening the following morning, though they no longer have the need to eat or sleep.
This campaign ended in January 2014, and details have been moved to this site at The Midnight Saga
Mythic Europe : The Twilight Saga
Mythic Europe is the default (and assumed) setting for Ars Magica campaigns. Based in Europe during the 13th century, it assumes a world where magic is real and the secretive Order of Hermes hides behind the scenes, balancing the power of human magic against the Divine, Infernal and older powers of the Fey.
The Twilight Saga was a campaign concerning a group of young magi who had stumbled upon an ancient power capable of changing the face of the world, and an ancient foe who would stop at nothing to regain control of that power.
Based in the Stonehenge Tribunal (England and Wales), the campaign ended when the Power was finally released.
World of Greyhawk
The World of Greyhawk is one of the oldest AD&D campaign worlds, and a strange setting to base an Ars Magica campaign in. However, Ars Magica was flexible enough to allow the addition of non-human races, high powered magic and monsters, demons and devils so loved of Greyhawk.
Some notes on the Greyhawk campaign are available.