The Grand History of Eberron (an impressive 499 page font of information about the Eberron campaign setting drawn from dozens of sources)
Eberron Wiki (not nearly as complete as one would like)
Abscission of Light (an Eberron Campaign with useful information)
After the Last War (an Eberron Campaign with useful information)
Zer0 Hit Points (Middle-Earth for 5e rules and monsters, also includes Human subspecies)
DNDBeyond for Races and Game Rule info
Giger's 5e D&D for additional races and Game Rule