https://donjon.bin.sh/ – Many great tools. Spell lists, magic items, treasure, monsters, etc.
https://kobold.club/fight/ – Combat encounter generator. Allows you to select from different source materials, official and 3rd party.
NPC Generator – A generator that includes, stats, traits, and a hook.
Tavern Generator – Everything you need to make a tavern on the fly.
Adventure Lookup – Categorized database of published adventures.
Matt Colville’s Running the Game – A treasure trove of good information for new Dungeon Masters.
The Angry GM – Lots of great articles to get the most out of your game.
5 Room Dungeon – A guide to building a 5 room dungeon that is simple, effective, and unique.
Cheat Sheets – Collection of cheat sheets. The combat sheet is particularly useful to hand out to your players.