Arcgis Hub (fast, recommended, but not real time)
Arcgis Hub live ( slow, real time sync with ESRI Server) ESRI arcgis portal limit each request return 10 sites, so you have 10, 20, 30........ until reach 4000 sites.
Most of city/county/state host their arcgis portal on domain hub.arcgis.com or opendata.arcgis.com. However, some of organization host their arcgis port other than ESRI arcgis.com (ex: use amazon cloud EC2). Those will NOT show on this listing, but will show on the other listing Esri open data 1000+ organization