In his inauguration homily in 1978, Pope John Paul II made it clear that the Church no longer makes the claim of possessing temporal power:

Pope John Paul I, whose memory is so vivid in our hearts, did not wish to have the tiara; nor does his Successor wish it today. This is not the time to return to a ceremony and an object considered, wrongly, to be a symbol of the temporal power of the Popes.

So he's not going to be using excommunication as a political tool; of course, in today's world where the Papacy has no significant military force, and with Catholicism itself ceasing to be a political entity, that's sort of moot anyway.

The power of excommunication still exists; for certain offenses it's an automatic excommunication (latae sententiae), such as heresy and apostasy, improperly disposal of the sacred host, (physically) attacking the Pope and consecrating a bishop without authorization.

For your final question, I'm not sure if it can be answered without violating the 20-year rule.

Bishops retain the power of excommunication. However, over the centuries, that has been reined in. As already stated, the pope no longer attempts to exert significant temporal power, so while in theory, he retains much more of his authority than one might expect, in practice, he does not use it.

The most notable developments in the power to excommunicate are the two legal codes of the Latin Church (the Latin Church is what most people think of as the Catholic Church, indeed, it comprises around 98% of Catholics, although there are Eastern Catholics, who do things more similarly to the Orthodox and other Eastern churches but with their patriarchs recognizing the pope as senior to them and with fully valid ecclesiastical authority, I don't really know enough about Eastern canon law to give a decent answer for the Eastern Catholics, but all popes since the Early Middle Ages have come from the Latin Church and you specifically asked about papal excommunication).

The 1917 Code of Canon Law, issued under the authority of the Pope Benedict XV, was the first official code of Latin Church law. Before this point, since there was no clear definition of an excommunicable offense, bishops had relatively arbitrary authority to excommunicate. Since this specifically defined what an excommunicable offense is, as well as what offenses warrant suspension (a prohibition on a priest celebrating sacraments) or interdict (a non-excommunication prohibition on receiving sacraments), bishops could only pronounce excommunication as a punishment for the listed things and not other things that people were excommunicated for before this came into effect.

1917 Code of Canon Law's section on excommunication (in Latin): http://www.intratext.com/IXT/LAT0813/_P88.HTM

A new Code of Canon Law came into effect in 1983, under Pope John Paul II.

Official text of the 1983 Code of Canon Law (Latin): http://www.vatican.va/archive/cod-iuris-canonici/cic_index_lt.html

An English translation of the 1983 Code of Canon Law (which u/Felinomancy linked to portions of): http://www.vatican.va/archive/ENG1104/_INDEX.HTM

Book VI of the 1983 Code has the current excommunicable offenses scattered throughout.

1983's Code eliminated a type of excommunication, Vitandus (as opposed to Toleratus, so all excommunications now are de facto Toleratus), which required Catholics to avoid contact with the excommunicate, similar to shunning among some Protestants such as the Anabaptists. With vitandus removed, there are three terms to undersand: Latae Sententiae excommunications are excommunications that one automatically incurs upon committing the act (essentially, one excommunicates oneself by committing the act). Ferendae Sententiae excommunications do not take effect until pronounced by a bishop. While most excommunications can be lifted under the authority of a local bishop, excommunications reserved to the Apostolic See require papal authorization to be lifted.