Most of the GIS vendors and/or their users host forums or discussion lists about their products. While few vendors have full-time employees monitoring them, you may see posts from staff. Typically these focus on advice for using the product. ESRI has extensive forums for products and industries. Autodesk hosts a number of groups as does Bentley. MapInfo's newslist is quite active, as is one for Smallworld (GE Power). Intergraph, its product GeoMedia, Manifold, Caliper's Maptitude also have newslists.
These lists focus on more general use of GIS (GISLIST replaced the "original" GIS-L some years ago as "the" generic GIS list), its use in a particular area like journalism. Sometimes these are created for a specific topic and flourish for while, then go silent.
USENET is a hierarchy of discussion groups that started early in the Internet's development. There are a variety of ways to access them, including dedicated newsreaders (software) but I prefer using my browser via Google Groups (originally, it was called DejaNews). Groups of interest to GIS folks include some of the CAD ones, comp.infosystems.gis and a few other relating to satellite navigation, and imagery. These have all seen less and less traffic over the years as newslists and forums take on more of the burden of communications.
Mirrors and Consolidators
Several websites maintain archives of lists so you can find them in one place. GeoCommunity does this, though I like the interface at Directions Magazine better. Both these organizations host lists, gratis, as a service to our community.
Other Websites and Blogs
As blogs (Web logs) mature, some community-built ones and some personal ones are becoming "must reads." The "collaborative geotech weblog" that I read is DigitalEarth.org. I also read Starhill.