Most of us work online now, and there is much material there to make us occupied for a while If you know about a online depository/library/archive, write to hps.cesee@gmail.com.
OVERARCHING:
Resource Center for Medieval Slavic Studies (RCMSS) - https://rcmss.osu.edu/resources
List of online Ressources for Historians: https://www.kathill.co.uk/coronalife/2020/3/19/books-in-the-ether-online-libraries-and-resources
Online Ressources for History Teachers, by Sarah Kenney: https://docs.google.com/document/d/1pCW8N8CW8DDPNEeaI86O5m-BuS8PhaILvHl3L8eaCNA/edit
Free Resources on MUSE During COVID-19: https://about.muse.jhu.edu/resources/freeresourcescovid19/
National Emergency Library: https://archive.org/details/nationalemergencylibrary?tab=about
AUSTRIA
Austrian National Library has ample options of online access. From dailies and journals in ANNO to poststcards in AKON, and of course books in ABO: https://www.onb.ac.at/en/digital-reading-room
GERMANY
Deutsche Digitale Bibliothek: https://www.deutsche-digitale-bibliothek.de/
Digitales Forum Mittel- und Osteuropa: https://www.difmoe.eu/d/
BELORUSSIA
National library of Belorussia: https://www.nlb.by/
CZECHIA
Czech national digital library for teachers and students of higher schools: http://kramerius-vs.nkp.cz/
Catalogue of archival materials, incl. digitized materials: https://www.historickefondy.cz/
Library of the Czech Academy of Sciences (for Czech citizens or people living in Czechia only) https://www.lib.cas.cz/27870/knihovna-online/
Manuscriptorium, Digital Database of Cultural HEritage Online: http://www.manuscriptorium.com/en
Stories of 20th Century (Post Bellum): https://www.memoryofnations.eu/en
GREAT BRITAIN
Free access to Academic textbooks on Cambridge Core: https://www.cambridge.org/gb/academic/covid-19-resources-and-information
Oxford Academic, History of Outbreaks Collection, https://academic.oup.com/journals/pages/history_of_outbreaks
HUNGARY
Digitized Parliamentary Database (DPD): https://dtt.ogyk.hu/en
Hungarian Electronic Library: https://mek.oszk.hu/indexeng.phtml
Hungarian Cultural Heritage Portal: https://hungaricana.hu/en/
Rare Books and Manuscripts Collection, Architectural plans, and Image Collection: http://dlib.ogyk.hu/
Akadémiai Kiadó (the publishing house of the Hungarian Academy of Sciences), books 1950-1990: https://www.szaktars.hu/akademiai/
POLAND
Federacja Bibliotek Cyfrowych, On-line collections of Polish cultural and scientific institutions: https://fbc.pionier.net.pl/
Polona.pl: Polish National Library digital: https://polona.pl/
Central Catalogue of Polish archival collections, incl. digitized materials: https://www.szukajwarchiwach.gov.pl/en/strona_glowna
RUSSIA
St. Petersburg Archives (free of charge from 23.03): https://spbarchives.ru/
National Digital Library: https://rusneb.ru/
SLOVENIA
Digital Library of Slovenia - Digitalna knjižnica Slovenije: https://www.dlib.si/?&language=eng
UKRAINE
See out special section "Ukraine History of Science, digital": https://hpscesee.blogspot.com/2020/03/ukrainian-history-of-science-digital.html (by Martin Rohde)
UNITED STATES
JDC Archives, Archives of the Jewish Distribution Comitee: https://archives.jdc.org/