The stanfordgis listserv -- GIS users at Stanford Discussion List

https://mailman.stanford.edu/mailman/listinfo/stanfordgis

GIS users can sign up for the Stanford GIS Listserv for the latest updates from the SGC on remote teaching & learning resources, training opportunities, and more.

Qgis-user -- QGIS User Discussion List

https://lists.osgeo.org/mailman/listinfo/qgis-user

This list provides for discussion on using QGIS, including troubleshooting. It serves as the primary user community support mechanism for Quantum GIS. To see the collection of prior postings to the list, visit the Qgis-user Archives.

The Google Earth Engine Developers Discussion List

https://groups.google.com/forum/#!forum/google-earth-engine-developers

This is the primary venue for direct support of Google Earth Engine. The forum has been around for many years and has an incredibly useful archive, as well as being populated and monitored by the GEE developer teams, themselves. Very active, with short waits for responses. 

GeoNet, the Esri Community

https://community.esri.com/community/help-and-feedback

The GeoNet Resource Hub is a place where you can learn how to leverage GeoNet to work better, stay connected, and share experiences. Here you will also find tips & tricks, tutorials, tools and FAQs.

GIS Stack Exchange

https://gis.stackexchange.com/

Q&A for cartographers, geographers and GIS professionals.