This is a list of the sites I turn to when I want to understand how something in the game works, how to run it well, or just for GMing inspiration:
Dungeoncraft – Professor Dungeon Master, with his various waistcoats, gives good tips on playing streamlined games with an old-school feel inspired by Mentzer D&D and the Warhammer FRPG. His approach to story structure is particularly insightful.
Questing Beast – Ben Milton has created something of a hub for the Old-School Renaissance in gaming, of which I am a big fan. In particular, check out his YouTube channel for reviews of OSR products (many of which are available for free) from all across the web.
Runehammer – The eclectic Hankerin Ferinale (I suspect not his real name) runs the game in a way that chimes with me – straightforward, stripped back and fair but brutal. In particular, check out his “5e Hardcore Mode” for ideas on honing D&D.
Sly Flourish – The self-professed “Lazy Dungeon Master” offers a real trove of advice and materials for designing and prepping awesome sessions.