Resources

Blogs

A Collection of Unmitigated Pedantry is written by a historian and looks at the depiction of history (including historic fantasy) in popular culture, and talks about how it would really be done. There is so much good stuff here, start with the Resources for World-Builders page.
The Alexandrian is a treasure trove of information, the article series on Node-Based Scenario Design and Advanced Node-Based Design have really helped me think about how to structure my games, and the series on Jacquaying the Dungeon has made my dungeons less boring.
The Angry GM has a style which grates on a lot of people (and I’m not a fan of it either), but if you can look past that there’s a lot of good advice here.
ars lundi is a game designer and creator of the legendary West Marches style of play.
Goblin Punch is the home of the Goblin Laws of Gaming, and great creatures like the False Hydra.

Dungeon Scrawl is a simple browser-based mapping tool. I often use this to draw out the layout of a building, then export an image and continue editing it in GIMP.
donjon has a huge number of generators, for all sorts of things.
The Fantasy Name Generator can be a bit stereotypical, but if you just need a name now in one of dozens of different categories, it’s got your back.
The One-page Dungeon Generator isn’t going to power your full campaign, but it can be a source of inspiration.

Matthew Colville‘s Running the Game series has been a great help to me, even though I’ve never run D&D. I particularly enjoyed the video on roleplaying.
Modern History TV is about the world of medieval England, and what it was really like.
Questing Beast reviews a lot of great OSR books.
Seth Skorkowsky reviews a lot of scenarios, makes great system review/overviews (like this great multi-part Call of Cthulhu intro).