Giordano Bruno di Nola is perhaps best known for being the most famous heretic actually executed by the Catholic Church, or failing that, his rather prescient idea that the stars might be far-off suns, with inhabited worlds circling them much like our own sun. However, in his own time, he was much more noted for his mystical brand of Christianity, which he modestly called Nolanism, and for the fact that he essentially forced the church to execute him, in a manner more reminiscent of the death of Socrates than the suppression of Galileo.
Bruno's mysticism was quite syncretic, cobbled together from odd bits of Neo-Platonism, Egyptian Mythology, hermeticism, alchemical ideas, the Jewish Cabbalism of Luria which was fashionable in Italy at the time, and various semi-suppressed Christian heterodoxies. Nolanism is fantasically complicated, and difficult to explain or even understand. Some claim that its focus on words and their meanings prefigures the modern discipline of semiotics and the works of James Joyce, other that its mystical re-interpretation of Platonic forms is the direct precursor of Jungian archetypes. Bruno has been described as the first post-modern thinker.
The Catholic Church's execution of Bruno for heresy was not a sudden and unexpected strike, but rather a long and complicated drama that, in its defense, the church made many efforts to avoid. When Bruno left Italy to publish his books, the church told him not to return. However, one of Bruno's enemies fed him a false story that the church was no longer actively seeking to suppress his work, and so he returned to Bologna. When he did, the relatively liberal officers of the Italian Inquisition (who were very, very different from their co-officiaries in Spain) felt that they had no choice but to arrest him. They assumed he would do the reasonable thing, recant, and they could deport him and claim a moral victory. However, Bruno simply refused to recant, which caused the church a great deal of consternation. He was held under comfortable house arrest for a year while the Inquisition scratched its head and wondered what to do. Bruno was given three papal audiences where the pope personally asked him to recant, all to no avail. Eventually, the decision was made that if he wouldn't recant, he would have to burnt at the stake, if only as an example, and Bruno entered history.
Thanks to Gorgonzola for pointing out to me that Bruno was executed well before Galileo's run-in with Inquisition, and not the opposite as I originally implied.