Micro RPG Catalog, a collection of tiny RPGs.
This Person Does Not Exist. AI-generated portraits, could be used for NPCs.
Roll20 is an online virtual tabletop with various tools.
The Little Details LiveJournal community can be very useful for unusual facts and information.
Abulafia, a collection of user-contributed random generators.
SCIgen, an automatic computer science paper generator.
Mathgen, an automatic maths paper generator.
Maps from OpenStreetMap can be downloaded in various formats
Soviet Military Topographic Mapping has information on Soviet military maps, including where they can be downloaded or bought