sci.*
Usenet's science hierarchy — where researchers argued in public with each other, and with anyone who had a modem and a theory of their own.
sci.* was created in the Great Renaming for science and engineering broadly defined: sci.math, sci.physics, sci.space, sci.med, sci.crypt. Before the web it was one of the few places where a student could ask a question and get an answer from the person who wrote the textbook. John Baez published his long-running "This Week's Finds in Mathematical Physics" in sci.physics.research.
sci.crypt drew the most attention outside academia. Cryptography was discussed openly there while the US still classed it as a munition, and the source code for RC4 spread through it in 1994 after an anonymous leak. Open groups also attracted cranks, and telling an unknown talent from a committed circle-squarer was a skill sci.math readers picked up early.