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.
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.