Am I missing something?
SEMCOG is great. Honestly some of the best metadata I've seen. I'm glad I could share it. Is it Common knowledge...I'm not sure. My brother's architecture firm in Detroit wasn't aware of it and until I showed them the light. More and more communities/counties/mpos are implementing Esri's Open Data Portal. I usually check there first and use OSM/Get Map Data as a backup.
I'm in the Detroit Metro area as well..:)
The SEMCOG is only south east Michigan. Is there a site like this for other areas in the USA?
You just have to search wherever your project is. I typically look first for the county with a search term like this - Cook County Open Data Portal GIS and then I'll try the MPO (i.e. CEMCOG) and finally the city.