gwern.net. Inevitably here.
qntm.org. Excellent writing freely available, particularly "Ra" and "There Is No Antimemetics Division." These stories are also available on Amazon, and are worth the purchase.
Fantastic Anachronism. Miscellaneous on science, replication, and literature.
Andrej Karpathy's Blog. Karpathy wrote some of the best explainers about deep learning, worked at OpenAI, and now works at Tesla. Very fun.
https://sifter.org/~simon/. A personal website whose contents are nearly entirely unknown to me, but which contains an excellent novella "After Life," in the style of Greg Egan et al.