At Portal Militärgeschichte, Markus Pöhlmann reports that a joint German-Russian digitization project has made available a substantial number of World War One–era German military documents at the Russian defense ministry’s central archive. There is also a digitized collection of the German secret services from 1912 to 1945.
Additionally, the multi-volume Der Weltkrieg 1914-1918 (The World War), published by the Reichsarchiv in excruciating detail, is now available digitally thanks to the Upper Austrian State Library in Linz. As Pöhlmann points out, this work continues to be essential for operational history because it was written on the basis of documents that were largely destroyed in World War Two.
Finally, the Austrian-Hungarian counterpart has been digitized in Linz too: Österreich-Ungarns letzter Krieg 1914-1918 (Austria-Hungary’s Last War).