Micro RPG Catalog, a collection of tiny RPGs.
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.
This Person Does Not Exist. AI-generated portraits, could be used for NPCs.
Generated Photos. Another source of AI-generated portraits.
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