Undocumented Cell options

Please add any more undocumented Cell options that you know to this answer.

WholeCellGroupOpener

John Fultz posted an answer which had an undocumented option

CellPrint[{
  Cell["Click to open the section", "Section", 
       System`WholeCellGroupOpener -> True],
  TextCell["Some text"]}]

This produces a section cell that if you click anywhere on it will open and close the whole section.