Science

Life in our Phage World – Gorgeously illustrated textbook and collection of essays about the most common organism on the planet.

The Sequences – Compelling essays on rational thinking and cognitive biases.

Sci-Fi

Ra – Novel in which magic is a new form of engineering, and the plot only escalates from there. Also check out qntm’s short sci-fi stories, all of which are wonderful.

Creepy

Ted’s Caving Page – Astonishingly terrifying story.

The SCP Foundation – Files from the confidential documents of the mysterious SCP Foundation and the anomalous artifacts it contains. Endless worldbuilding and stories from a collaborative community.

Invisible Games – Short, beautiful, game-inspired creepypasta.

Alice Isn’t Dead – Eerie podcast about a truck driver looking for her missing wife and encountering a lot of supernatural shit along the way.

Lovecraftian

The King in Yellow – Great-grandfather of the “eldritch fiction” genre.

The Northern Caves – Unique Lovecraft-inspired novella set among early internet fan culture.

Move Under Ground – Astonishingly good fusion of Lovecraftian canon and Jack Kerouac.

Fanfic

Harry Potter and the Methods of Rationality – Rationalist Harry Potter fanfiction.

Republic of Heaven Community Radio – A brilliant Welcome to Night Vale / His Dark Materials crossover.

Webcomics

Goodbye to Halos – Beautiful beautiful queer modern-ish fantasy. The main character is one of the best I know.

Dumbing of Age – A fundamentalist Christian girl goes to college. Great writing, great characters, lots of humor and drama, updates daily. Warning that you may get sucked into reading several years of updates in one multi-hour sitting when you start.

Other

Nevada – Extremely compelling novel about a trans woman in New York City.

Unsong – Novel about a world where the Torah is verifiably true, the world is ending after the Apollo mission cracked open the crystal sphere containing the fixed stars, and corporations are using sweatshop labor to discover the hidden names of God.

Awful Hospital – Creepy, gross, and hilarious webcomic about a woman who wakes up in a terrifying nightmare hospital, with her son missing somewhere in it.