The National Library of Israel

From the website:
"The Jewish National and University Library was established in Jerusalem in 1892. Over the years it accumulated both ancient and new works pertaining to Jewish intellectual heritage, as well as books published in various parts of the Jewish world. When the Hebrew University of Jerusalem was established in 1925, the Library was officially named "The Jewish National and University Library" and the scope of the collections was greatly expanded.
Until 1948, the Library was located on the Mount Scopus campus of the Hebrew University of Jerusalem. After the War of Independence, the Library was relocated several times within Jerusalem until it moved to its current location on the Givat Ram Campus of the Hebrew University of Jerusalem in November, 1960. 
In July 2008, subsequent to the enactment of the National Library Law of 2007, the Library was renamed "The National Library of Israel". In 2012, the National Library will celebrate its 120th anniversary. In keeping with its dynamic history, the Library continues to change and evolve. The Master Plan for Renewal 2010-2016 includes construction of a new home for the Library in Jerusalem's national precinct (Kiryat HaLeom)."

Among its holdings are many Yiddish publications. The main catalogue can be accessed here.