A list for those who may not yet know. In no particular order. I'll also post a short biography of each name. Please let me know if I omitted anybody.
FR = The Frierdige Rebbe, Admor Rav Yosef Yitzchok of Lubavitch, Nishmoso Eden. Born 12 Tammuz 5640, passed away Shabbos 10 Shvat, 5710. Succeeded by the current Rebbe. Also known as the Rebbe RaYaTZ.
RCL = Reb Chaim Lieberman, Secretary of the Rebbe RaYaTZ of Lubavitch.
RSAK = Reb Shlomo Aron Kazarnovsky. Talmid of Tomchei Tmimim in Lubavitch, Rav in Brooklyn, and distinguished chossid of the last 3 Rebbeyim of Lubavitch.
RSG = Reb Shmaryahu Gurary, eldest son in law of the Rebbe RaYaTZ of Lubavitch and Executive Director of the Lubavitcher Yeshivah Tomchei Tmimim in New York. Passed away on 6 Adar Rishon 5749.
RYH = Rav Yitzchok Hutner, Rosh Yeshivah of Yeshivas Rabbeinu Chaim Berlin and author of the Pachad Yitzchok series. Died 20 Kislev 5740.
RSC = Reb Shlomo Carlebach, former Mashgiach of Yeshivas Rabbeinu Chaim Berlin. Not to be confused with his cousin the entertainer.
RSF = Reb Shlomo Freifeld. Talmid of RYH and founder of Sho'or Yoshuv institutions in Queens.
RAK = Rav Aron Kotler. Rosh Yeshivas Kletzk and Beis Medrash Govoha in Lakewood, New Jersey. Active in many different important Jewish causes in America as well as abroad. Passed away 3 Kislev, 1962.
RMF = Rav Moshe Feinstein. Noted Posek in New York and author of the responsa Igros Moshe. Rav in Lyuban, Russia, passed on in 1986.
RAS = Reb Aron Schechter, Product of East New York, Brooklyn. Rosh Yeshivas Rabbeinu Chaim Berlin.
RJBS = Rav Yoseph Ber Soloveitchik, prince of Brisk. "The Rav." Rav in Boston and Rosh Yeshivah of RIETS, Rav Isaac Elchonon Theological Seminary. Died 1994.
RASol = Rav Aron Soloveitchik. Brother of RJBS, son of Rav Moshe, son of Rav Chaim, and Rosh Yeshivah of Brisk - Chicago.
REW = Reb Elchonon Wasserman, HY"D. Talmid of Rav Chaim Soloveitchik of Brisk and the Choftez Chaim, and Rosh Yeshivah in Baranovitch. Killed by the Nazis in Kovno, 1941.
BG (SBG) = Bere, Ber'ke Barry Gurary. Only grandchild of the FR. 1923-2005.
RSG = Rav Shimon Groner. Succeeded RSC as Mashgiach in YRCB.
RYD = Rav Yonoson David. Son in Law of RYH, Rosh Yeshivah (RY) of Yeshivas Pachad Yitzchok, Jerusalem.
RAM = Rav Avigdor Miller. noted author and thinker. former Mashgiach of YRCB (Yeshivas Rabbeinu Chaim Berlin.)
RSK = Rav Shmuel Kamenecki. son of Rav Yaakov Kamenecki, and Rosh Yeshivah of the Philadelphia Talmudical Academy.