Europeana is a European Union funded project allowing access to "millions of digital items provided by Europe's museums and galleries, archives, libraries and audio-visual organisations".
The number of Yiddish sources is at the moment limited but set to grow as Europeana expands. This search will give you all the content that is Yiddish related. Please note that there are many relevant materials for Jewish history to discover by searching with other terms.
Europeana includes content from the Digital Warsaw University Library which offers a number of Yiddish-language materials online. The full list of currently participating institutions can be found here.