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.
RPG Market Generator – Randomly generates shops, inventory, and NPCs. Created by u/Aradreth_81.
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.
DM’s Guild & Drive Thru RPG – Official Wizard’s of the Coast websites for 3rd party content.
5 Room Dungeon – A guide to building a 5 room dungeon that is simple, effective, and unique.
DnD Speak – Categorized lists of everything to add to your campaign. Add your own content at r/d100.
Table of Tables – Random tables, courtesy of r/BehindTheTables.
Cheat Sheets – Collection of cheat sheets. The combat sheet is particularly useful to hand out to your players.
Shop Catalog – Different inventories based on shop type, size of town, costs etc. Created by u/jrobharing.