I was trying to add a custom command to one of the Editor's context menus (SketchToolContextMenu). Easy enough if you know the GUID or the ProgID, but in this case I was going crazy trying to find either. None of the documents I came across seemed to list the particular context menu that I needed. So I tried to enter some ProgIDs that seemed likely, e.g. "esriEditor.SketchToolContextMenu", but didn't have any luck.
Then it occured to me (duh...) that the registry should have an entry. Opening regedit, navigating to the ESRI folder at HKEY_CURRENT_USER\Software\ESRI and doing a Find on "SketchTool" yielded the following registry key:
Although the easily readable ProgID is nowhere to be found, at least the Guid was listed. From there it was an easy copy and paste into my custom extension, and now my custom command shows up in the correct place.
Oh, and by copying the Guid value and doing another Find, eventually you can find the ProgID: "esriEditor.SketchToolMenu"
(I would have expected "context" to show up somewhere in the ProgID...no wonder my attempts at guessing it didn't work.)