Louis XVI and Nicholas II were overthrown; the forces that overthrew them referred to them by common names as a way to humiliate them.
The Netherlands has a long tradition of monarchs abdicating in favor of their children, and in fact Queen Beatrix recently announced her intention to do so. When she does she will still be Her Majesty, however, and will be known as Queen Beatrix the Queen Mother.
Under the British system (generally the most familiar to Americans) monarchs are referred to as His (or Her) Majesty, abbreviated HM, and referred to as "Your Majesty." There are no specific rules governing titles and styles on the event of a monarch's abdication, because such events are exceedingly rare and, throughout most of history, generally accompanied by the ex-monarch having their head chopped off. The one time it happened in modern history was 1936, when King Edward VIII abdicated. At that time it was decided that he would revert to the style His Royal Highness, abbreviated HRH. This was done because, although he was no longer king, he was still the son of a king, and therefore a prince of the blood. The children and grandchildren of a monarch are always royal, and therefore HRH; this cannot be taken away. The same is true for titles of nobility, unless Parliament has passed an Act of Attainder, these titles and styles cannot be taken away.
(Interesting side note: The child of Prince William and Kate Middleton would not, technically, be royal upon birth. This is because he or she would be the great-grandchild of a sitting monarch, and therefore one generation too far from the throne for royal status. However, the Queen will issue letters patent before the child's birth bestowing royal status, and therefore the HRH, so that the child of the heir to the throne will not be a commoner!)
The papacy is an unusual institution in that it resembles a royal court in many ways, yet its leader is elected. My suspicion is that, as an honorific, Benedict will either retain the style His Holiness or be assigned some other, unique title and style to represent his status as a former pope. If not, however, then according to Canon law he would revert to being simply Bishop Joseph Ratzinger (with the style His Excellency) on 8pm February 28, 2013, when his resignation takes effect.