I recently switched to the
Gazetteer of the Known Realms (Dungeon Crawl Classics #35)
using the Wilderlands of High Fantasy for some of the city details
I highly recommend the combo!
Regios, Spell Thematics, and the Order of Hermes from Ars Magica
Vampire Clans and Ghoul families from White Wolf
Sigil and Factions from Planescape
Organizations, Nobility, religion, BBEGs from Ptolus (basically all of Ptolus....)
FC1 and the demonomicon articles in dragon
Iuz from Greyhawk
Daelkyr, Changelings, Shifters, and artificer from Ebberon
Sha'ir from Dragon Compendium and Al Qadim
Shelzar, City of Sins, from Scarred Lands
Ancient Kingdoms Mesopotamia by goodman games
Sanctuary, the place, some of the history (Thieves World)
BBEGs, and the Black Company from the Black Company
Shadowmagic, Skullport from Forgotten Realms
Thrikreen and merchants from DarkSun
That is just some....
of the above I think the Regios and Spell Thematics from Ars Magica are usually the biggest difference people notice in my games...the fact that there are places of power in the world where magic works differently (Regios - think Planar Touchstones turned up to 11) and that all the spells look/sound/smell different than what the players are use to (because they are unique to the spell designer) creates a different atmosphere at the table.
If I could get spontaneous casting from Ars Magica I'd be very happy....but as of yet I haven't been able to get my player's mentally to accept some the offered conversions