I have posted about this before but I am sharing this again since there is some new info that can help people find ArcGIS data.
The PDF I curate is at https://mappingsupport.com/p/surf_gis/list-federal-state-county-city-GIS-servers.pdf
Once a week my code scans each address and any bad links are either fixed or flagged. An updated list is usually posted each Wednesday.
The new info is that another developer is building a tool that lets you search these servers based on a keyword. See https://blog.gisdata.io/
