The digital library of Warsaw University Library offers a number of Yiddish materials, including the only run of a historical Yiddish periodical avalaible online at the moment, Di Kehile Shtime.
This search will yield all currently available Yiddish materials. Please note that there is much relevant material for Polish-Jewish history in Hebrew and Polish as well.
To view items you need to install the DjVu plugin.
The contents of the digital Warsaw University Library are also available through Europeana.