HârnWorld is a realistic setting, based on the reality of medieval society and culture, blended with the fantasy elements.
+ http://columbiagames.com/intro-to-harnworld-2/
+ https://en.wikipedia.org/wiki/H%C3%A2rn

Mystara is the default setting for the Basic DnD in 1980-1990.
https://en.wikipedia.org/wiki/Mystara

Calidar is a setting from the Mystara creator. He made it when he didn't succeed to buy copyrights on Mystara.
https://bruce-heard.blogspot.com/p/a-word-about-calidar.html

Wizards of the Coast in 2002 run open design competition to develop brand new fantasy campaign setting. The winner is Eberron, a fantasy world with steampunk, magic and number 13:
https://eberron.fandom.com/wiki/World_of_Eberron

Other finalists are unknown because of copyrights:
https://www.reddit.com/r/DnD/comments/50jl4l/when_wotc_created_eberron_what_were_the_other/

Bark Sun is a post-apocalyptic fantasy. Sic! Most of the races are dead, halflings are savages. If you need something original and brutal, it's for you.
+ http://www.athas.org/
+ https://en.wikipedia.org/wiki/Dark_Sun

GM Tips - collection of good advice from Matthew Mercer how to run DnD campaigns:
https://www.youtube.com/playlist?list=PL7atuZxmT9570U87GhK_20NcbxM43vkom

Critical Role is a show where Matthew Mercer runs a DnD campaign.

Exandria is the world where this campaign runs. It's a homebrew of Mercer. In 2017, book "Tal'Dorei Campaign Setting" about the setting was published. So, you can run campaign in the same world!

In 2020, Explorer's Guide to Wildemount was published by Wizards of the Coast, owners of DnD.

The Order of the Stick (OOTS) is a comedic webcomic that satirizes tabletop role-playing games and medieval fantasy. The comic is written and drawn by Rich Burlew, who illustrates the comic in a stick figure style.
https://www.giantitp.com/comics/oots.html

Overly Cautious D&D Party Still in Starting Tavern After 10 Sessions
https://thehardtimes.net/harddrive/overly-cautious-dd-party-still-in-starting-tavern-after-10-sessions/

A guide through all the starting equipment in DnD:
https://www.youtube.com/watch?v=doWAdSPfgdI

When your druid turned into a tree at the middle of the battle

Why a sword is a bad choice to fight a giant monster:
https://youtu.be/LTUPQDpO5_A

A small Print and Play games collection:
https://print-and-play.asmodee.fun/

Character sheet builder for dnd 3-5, pathfinder 1-2, starfinder:
https://www.dyslexic-charactersheets.com/

Micro RPGs:
https://github.com/brunobord/micro-rpg-catalog

OSR (Old school revival) is an interesting movement in tabletop RPGs world. If I understand correctly, the idea is to focus on believable and interesting action rather than crunchy numbers and balance, making easy to generate adventures on-the-fly:
+ https://en.wikipedia.org/wiki/Old_School_Revival
+ https://www.reddit.com/r/rpg/comments/7a8kqm/

Stars without number is an OSR RPG in spaaaace:
+ https://youtu.be/lVTu69Usu2Q
+ https://youtu.be/ZyAMLPj4N0o

List of retroclones (osr rpgs)
https://1d4chan.org/wiki/Retroclone