Three of the ‘Ten Martyrs’ were killed on this date: Rabbi Chanina S’gan HaCohanim, Rabban Shimon ben Gamliel and Rabbi Yishmael ben Elisha. (this is brought down in the Tur and Shulchan Aruch OC 580). Their deaths are remembered in the kinah of Tisha B’Av ‘Eileh Ezkera’.

Rabban Shimon ben Gamliel was the grandson of Hillel. He was the nasi durign the end of the Second Temple period and after the destruction, at the time when Rabbi Yochanan ben Zakkai was the Rosh Yeshiva. His grandson was Rebbe Yehuda HaNasi.

Rabbi Yishmael, Cohen Gadol went to learn Torah from Rabbi Nechuniah ben Hakanah at the age of 13. He was able to speak to angels, and when the Sages heard the decree that ten of their number were to be killed as atonement for the sin of Yosef’s brothers it was R’ Yishmael who ascended to Heaven to hear whether this was decreed from on High.
He was very beautiful, and the Talmud tells that when he was being taken out to be killed, Caesar’s daughter saw him and asked that his skin be flayed from his face and made into a mask so that she could gaze at his beauty (Avodah Zarah chapter 1).
The Talmud in Brachot (7a) tells that he used to go into the Holy of Holies each Yom Kippur and would be escorted in and out by an ‘old man’. G-d asked R’ Yishmael to give Him a blessing, which was turned into the well known song ‘tanya’
He is credited as the author of the mechilta (a halachic commentary on the book of Shemot).

Rabbi Chanina S’gan HaCohanim (deputy cohen gadol) appears in Masechet Avot and says (chapter 2) ‘pray for the welfare of the governement for if it were not for the fear of them, each person would swallow his neighbour alive.