MathOverflow is almost ten years old now. Soon (if not already) the webpage will be old enough that some of the users may not have memories of the internet before MO. What were favorite (internet based, say) substitutes for MO before 2009 of current users?
Before MathOverflow, the culture of mathematics question-and-answers was quite primitive.
I remember the days when a student would impress their mathematics questions, expressed in cuneiform, by pressing with a stylus into clay tablets, and then bake them and carry them the next day to the summit of the mountain, to be viewed the next dawn by the gods. The priests would capture the birds at the summit who had seen the tablets, and divine the answers by inspecting their entrails.
In a major paradigm shift, it became common practice to write one's cuneiform in a transliteration of LaTeX, which led to far higher quality entrail output. Such students were the envy of their peers.
In later days, I had to march ten miles uphill in both directions through the snow, first depositing the computer tape containing my question at my supervisors pigeon-hole, and then later retrieving the dot-matrix printed output.
Finally, an oral tradition arose, whereby the students would ask their questions into the midst of a lecture, hoping for some kind of answer in signs. Oh, a wondrous day it was, when the answer to one's question appeared written on the chalkboard during the lecture!
IRC (Internet Relay Chat) had #math channels where people discussed concepts using an obscure dialect called ASCII.
There were moderated and unmoderated forums with thread based email discussions called USENET news. Some of them dealt with mathematics, and were remarkably free of spam and noise before some corporation bought them and made them publicly available (and opened the spam floodgates).
There were (and are) efforts by small groups to communicate by email and by web pages about various subject areas of mathematics. Although it was started earlier, by 1996 there was a repository of submitted scientific articles, some of which included mathematics, at Los Alamos National Labs. (Later Cornell took it over. It's called ArXiv.)
Some people shared their list of hyperlinks for internet resources. Occasionally a company would incorporate those lists into their curated databases. One of these companies was Yahoo.
The Wayback Machine has records of some of these pages. You might take a nostalgic tour one of these days.
Oh, and there really was no substitute for MathOverflow, primarily because the question answer format combined with a form of mathematical markup was still coming into its own. Also, there was no moderated group with such rapid turnaround as well as a scheme for community moderation. Living in caves and clubbing one another is not far from pre MathOverflow.
Gerhard "Hasn't Looked Back That Often" Paseman, 2018.12.10.
The Categories mailing list has been already mentioned in comments, I just want to add one detail, I think a significant one.
Mac Lane almost never posted there. I know several people that were sort of disappointed. Don't remember who told me this, but, when asked about it, he responded that such kind of communication was for birds.
He thus foresaw birth of Twitter many years ago.
The Secret Blogging Seminar used to have a Requests thread, where people could suggest topics for us to write about.