This is amazing:
https://youtube.com/playlist?list=PLlUk42GiU2guNzWBzxn7hs8MaV7ELLCP_
It starts from how to run the first game and proceeds to all related topics, like improvisation and alignment.

Funny stories from DnD campaigns told and animated by the DM:
https://youtube.com/c/puffinforest

Project "Atomic rockets" brings "science" into "science fiction". It tells about space travel and spaceships from the science perspective, gives some basic terms, formulas, estimations. And there is a lot of text.

The best place to start is "common misconceptions":
http://www.projectrho.com/public_html/rocket/misconceptions.php

Catalogue of 2665 adventure scenarios, fantasy only, mostly dnd 5e:
https://adventurelookup.com/adventures
The idea is to have nice filters to find what suits your group best.

How to make witcher-like monsters:
https://youtu.be/GhjkPv4qo5w

How to make a small 20 century town map:

1. Search for a small town that fits what you want. Some names can be found on wiki, r/kidsonbikesrpg, r/gravityfalls.

2. Download Google Earth desktop version. The web version won't do.

3. Find the city you selected. Search by name didn't really work for me, so I copy-pasted coordinates from the wiki page.

4. Check "View / Historical Imagery" and go all the way back to the year you want. For the city I selected, there were awful images from 1985 and quite nice black and white images from 1994 which is not bad.

5. Use "Add Placemark" to annotate the map.

6. Use "File / Save / Save Image" to export the result.

7. Also, there are great vintage high resolution maps at https://www.oldmapsonline.org/.

Yesterday Kevin Crawford, author of Stars Without Number, released the free version of Worlds Without Number:
https://drive.google.com/file/d/14qSGink8i1Y4ULaqdivrcvyKNI6Rkbes/view
I've scanned it through and it looks like the best fantasy ttrpg I've ever seen! At least, it presents a world I can believe in. Most of the other games leave too many questions how all this superpowerful magic didn't tear everything apart. And since it is OSR, you can play with it any classic dnd adventure.

The Hobbit trilogy cut into 4 hours movie:
https://thehobbitthecardinalcut.wordpress.com/
Motivation:
https://youtu.be/XB6h9uCAZmI

Wikipedia has quite an interesting article on attributes in different rpg systems:
https://en.m.wikipedia.org/wiki/Attribute_(role-playing_games)

A small post from me about the role of DRM in making the apocalypse in my setting:
https://www.reddit.com/r/SWN/comments/m93310/drm_as_the_reason_for_silence/
Some people seem to like it. I'm happy that my small idea will end up in campaigns of other DMs.

A random midnight thought: does anyone use lego as dnd terrain? Yep, of course, some do:
https://youtu.be/wBAkk_DmU7M

Time (in theory) to get close to speed of light with 1 g:
https://setiathome.berkeley.edu/forum_thread.php?id=34381

> Observed from the spaceship, accelerating at 1g would reach 0.77c after 1 year.
Observed from Earth, it would take 1.19 years, and would have travelled 0.56 light years.

In case you need to role play an engineer:
https://en.m.wikipedia.org/wiki/Turboencabulator

A few good examples:
+ https://youtu.be/MXW0bx_Ooq4
+ https://youtu.be/aW2LvQUcwqc

What happened in 80's:
http://www.thepeoplehistory.com/1980s.html
Handy if you're about to run a game in this time.

A book about how aliens and alien worlds could look like:
http://www.xenology.info/Xeno.htm

Funny lotr art:
http://www.corblok.com/

Constructor of random generators and a huge collection of existing ones:
https://perchance.org/welcome
There is no sorting or convenient search, so it's hard to find good ones. But there are good ones. Their subreddit has some links:
https://www.reddit.com/r/perchance

A nice constructor of the character appearance, with pixel art items:
https://reroll.co/
Also, character sheet manager for DnD 5e. The free version art is very limited, though, and premium is a little expensive. However, it still can be a good inventory manager.