Play by Post
You can think about Angry GM’s attitude and writing style what you want but he knows how to break down game mastering. This article about the big picture is a must read. Feel free to skip the intro if you don't like long-winded text.
Want to introduce new players to RPGs? Drunkens & Dragons have a YouTube video about Fluid Fundamentals with several tips. Ignore the drinking and burping, the content is good. I will post a summary of the video in a few days. The channel looks like it contains some gems. Check it out here.
Free audio books from Appendix N authors from Librivox.
Codex Dark from Gauntlet , a free issue of an indie zine. Horror themed.
Ruination (A SCRAWL dungeon), a dungeon generator for Ruins. PWYW.
RND issue 4, the make-your-own-adventure zine! Features towns, ruins, wilderness, and fresh air for your adventurers to explore. Great for solo role-playing and Game Masters. PWYW at drivethru or at gumroad. Physical copies + pdf and previous issues at thunderbunnypress.com. I love this zine.
A handy One-roll Dungeon Generator (Google Spreadsheet) is here.