These include museums, memorials, and Jewish historical institutions.
The acknowledged primary resource in the U.S. is the Center for Jewish History including multiple partners, all housed at the same address in New York City. The most notable of these for our purposes is YIVO Institute for Jewish Research.
Also in New York City: The Dorot Jewish Division of the New York Public Library offers a large collection of Judaica.
The primary resource in Israel for finding information about Holocaust victims is Yad Vashem, The Holocaust Martyrs' and Heroes' Rembrance Authority
The Family Tree of the Jewish People and the database of the Douglas E. Goldman Jewish Genealogy Center are accessible to visitors to Beth Hatfutsoth -- The Museum of the Jewish People, Ramat Aviv, Tel Aviv, Israel
The National Archives of the Republic of Belarus maintains regional collections in Brest and Grodno --of primary interest to Brest researchers-- as well as others in Gomel, Minsk, Mogilev, and Vitebsk. (Apparently some Brest-area records may be found in the Minsk archives, as noted in our page, here.) The Archives offer this summary of their genealogy resources. This includes documentation of a long-time collaboration with the Miriam Weiner Roots to Roots Foundation
See also this JewishGen Article, Belarus Records in Various Archives.