Pope Formosus was succeeded by Pope Boniface VI, who died of gout after 15 days. He was succeeded by Pope Stephen VI (VII) (all "Stephen" popes had a dual number because it is uncertain exactly how many of them there were), who staged the infamous Cadaver Synod in January of 897 (9 months after Formosus had died).
In this disgusting spectacle, the corpse of Formosus was dressed up in full Papal gear, sat upon a throne, and subjected to a mock trial. The dead pope was found guilty of perjury, coveting the papacy, and having violated canon law by transferring from one diocese to another (before becoming the Pope (the bishop of Rome), he had been bishop of Porto).
All of Formosus' acts as Pope were declared null and void, and the three fingers on his right hand with which he had given blessings and swore oaths were cut off. He was reburied in a common grave, and then exhumed a second time, and thrown into the Tiber River.
A hermit found the body and gave it a proper burial, and Pope Theodore II ended up exhuming the body once again and giving it a proper Papal burial at St. Peter's (the original).
Stephen VI (VII), after the trail, required all clergy appointed by Formosus to turn in letters declaring their ordinations invalid. The people rebelled against him, and he was stripped of the papacy, jailed, and then strangled to death for his actions.
Why did Stephen feel he needed to hold the Cadaver Synod? 1) Formosus had already been bishop of one diocese, so it was technically illegal for him to become Pope. But Stephen, too, had become Pope after being Bishop of Anagni. But he was appointed as such by Formosus. If Formosus was never really Pope, then Stephen was never really a bishop, and thus his papacy couldn't be challenged on those grounds. 2) He was crazy and full of hate in a time of intense rivalry, and he took his opportunity for revenge too far, not realizing how drastically it would backfire.
Notice that there has never been a Pope Formosus II.
A further note on Gamaliel's information re: the number of pope Stephens. Unconsecrated popes have never been considered popes, and they still aren't. The original Stephen II died after election to pope, but before consecration. The official Vatican directory of the Popes, Annuario Pontificio, listed him as a pope anyway until 1961. Then they took him off the list.