• “Visual Studio Integrate” MSDN Forum
  This is the preferred forum to ask questions about Visual Studio extensibility. I and many Microsoft members of the Visual Studio extensibility and SDK teams visit this forum daily to answer questions.
• Stack Overflow
  Use the appropriate tags when posting in this Q&A site such as vsx, vs-extensibility, vsix, vspackage, visual-studio-addins, envdte, visual-studio-sdk, etc.
• Visual Studio Extensibility UserVoice
  The site is for suggestions and ideas for the extensibility of Visual Studio.
• Gitter – Microsoft/extendvs
  Community for developers of Visual Studio extensions.
• Gitter – dotnet/roslyn
  The .NET Compiler Platform (“Roslyn”) provides open-source C# and Visual Basic compilers with rich code analysis APIs.
• Gitter – Microsoft/VSProjectSystem
  Documentation for extending Visual Studio with new types of projects.
• Gitter – ligershark/side-waffle
  A collection of Item- and Project Templates for Visual Studio

Archived forums

• Visual Studio .NET Add-Ins Yahoo Group (archived)
  This used to be the best forum for answers about .NET add-ins during the years 2002-2004 but now it’s quite dead. However, the Message history is a good resource to find answers of past questions. Free membership.