(off the top of my head, so possibly mistaken)
Jeff Baker
Military History of the Eternal Empire
Dave Modiano
Modifications of the rules for Thresh, thresh card drawings, legends of the Sun and the Moon, the Story of Brijghalod, the Man who Cheated Death, the constellations (not on line yet) and the huge Map of City Tzalmir that I have finally managed to scan in (as well as its companion document, Fodor's Tzalmir). This man clearly has too much time on his hands.
Mike Person
Thresh Scrying meanings, essay on prophecy, legends of the War Beasts, the Lovers' War, the Cult of the All-Father.
Syndic Economic Reports
Cael MacAyre
Legends of Yontzu Manfoglio and the Accidental War, and all the longest dictionary entries.
Charles Hope
Original Thresh Card Rules, the legend of the Glass Mage, the legend of the Ruby of Flame, several countries.
John Conger
Legends of the Mad Earl and the Summer Winter, essay on the Masks of Alkynia
Jerry Marty
Teachings of the Church of Shagras, Easterner quotes, and the Memoirs of Brother Finnbogg
typing in the Ars Magica spells, which I haven't put on the net for fear of being eaten by copyright lawyers, and the legend of the Temple of the Broken Plate. Triellian naming customs and brine wool.
Nick Carter
Tzalmir man pages (not on the web), and the legend of the Shadows of Adjudicator, and the Alchemy documents.
Kate Mahoney
All the artistic gifs, the legends of the the Heralds of Tzalin, Flower of Ithyra, the Last Herald, several countries.
Megan McCallion
The Crab's Tale, and the Road to Vendilon.
Andrew Shultz
The Other Gods
Marleigh Norton
The Lost Eye
Legend of the Red Queen. Actually, I don't know who wrote this one.