Heroes III Wiki - small server for questions about the wiki
Horn of the Abyss - official international server for Horn of the Abyss (created by the administrators of the online lobby)
Heroes of Might and Reddit - largest English server for Heroes of Might and Magic
Might & Magic
Might & Magic: Heroes 5.5
Sok z Crag Hacka
Kebab League - read more here
Feel free to add any HoMM related servers to this list.