For some reason, the list below (thanks Jonathan) was eluding me when I was creating my "OpenInBlend" context menu for Visual Studio 2008. I post it here for posterity, as well as hoping others find it easier to locate than I did:

  
  public readonly static string Standard = "Standard";
  public readonly static string ClassViewItem = "Class View Item";
  public readonly static string WebItem = "Web Item";
  public readonly static string MultiItem = "Cross Project Multi Item";
  public readonly static string WebFolder = "Web Folder";
  public readonly static string ProjectNode = "Project Node";
  public readonly static string Item = "Item";
  public readonly static string Folder = "Folder";
  public readonly static string Project = "Project";
  public readonly static string Solution = "Solution";
  public readonly static string MenuBar = "MenuBar";
  public readonly static string ToolsMenuBar = "Tools";
  public readonly static string StandardMenuBar = "Standard";
  public readonly static string CodeWindowMenuBar = "Code Window";

 

VSIP Addin Custom CommandBars - jonathan's blog