Procgen
Watabou – The absolute best place for procgen cities, towns, dungeon maps, and so so much more. All Free, beautiful styles, customizable. Oleg Dolya is the God of the procgen world, honestly.
Donjon – A shitload of awesome random generation tools. Maps, random tables, shops, adventures, towns, and the home of the wild endless oubliette.
OSR
Hexroll – Procgen entire worlds, entire campaigns, dungeons, basic VTT capabilities. Honestly just…go check it out, zoom in, blow your fucking mind. This is one of the wildest things I’ve come across.
Cyberpunk
Cybersmiley’s Datafort – “an unofficial fan site dedicated to the R. Talsorian’s tabletop RPG Cyberpunk 2020 and Cyberpunk Red. Here you will find articles, web page apps, and other unofficial resources for CyberPunk 2020/Red.”
Red Data Generators – “Generate names, night markets, nomad markets, and weather for use with Cyberpunk Red.”
DND
Druid Wildshape Spreadsheets – Super handy druid wildshape tables that make wildshape slightly less annoying.
The Monsters Know What They’re Doing – Rightfully well-known and loved articles regarding combat tactics and accurately roleplaying monsters.
Custom Map Making
Dungeon Scrawl – Custom map maker, you can create from scratch or even upload json’s from watabou or donjon. Free or Subscription. Also has a pretty cool discord community 😉
Helpful Authors/Blogs
Sly Flourish Site – Author of the Lazy DM series. Amazing advice and helpful, thought provoking info for any budding or veteran DM.