the Vaults of Pandius, the official Mystara fansite. I have compiled a list of the most comprehensive of these fan productions here.
This week, a new community release was announced at the Piazza. This is the History of Dragonkind, by Simone Neri. Over the years alot of material on Mystara's dragons and what makes them different from the dragons of other D&D settings. Neri has been able to combine all of these sources presenting a great overview of the history of dragons from before the Blackmoor Era, through the events from the Dragonlord Trilogy and up to the modern Mystaran Era.
You can download this excellent fan sourcebook here.