DM TRICKS & TRAINING
Encounter builder tool with list D&D 5e Monsters with advanced filters and to target for number of players and party level.
Encounter generator with ability to set difficulty, environment and more.
My favorite Spell Reference Site: https://www.dnd-spells.com/spells
EXTRA: Printable Spell Cards
Bad formatting, but the advice is helpful. Just copy and paste it somewhere.
Giant In The Playground forums! All about tabletop roleplaying, and a huge community. Not to be missed!
Many articles about different RPG designs, RPG theories, Indie RPGs, etc... I didn't read through it, but it looks massively interesting.
A couple of articles on stances (Author, Actor and Director). I didn't take the time to explore the website yet, but it looks good.
I use this when i have an idea but feel it is not interesting enough. Enter the Trope.
Combination Tool - Random Quest; Magic Item; Treasure generator; etc
Story Telling Skills for the GM
Masterplan has been my go-to thing when I want to design a long series of events, creating a quick flowchart