alt Hacker NewsI used to frequent cs.rin.ru for all things non-steam back when I operated non-steam CSS servers.
UnknownCheats is also absolutely amazing for cheat development. Back when I was writing undetected kernel cheats for my own experimentation purposes, I learned so much there.
I made my lifelong best friends hosting non-Steam servers, and writing the first cracks in Lua to generate fake Steam IDs from IP addresses.