alt.folklore.computers
The Usenet group for computing history, where veterans of the punch-card and mainframe era explained how things really went.
It grew out of the alt.* hierarchy around 1990 as a computing spin-off of alt.folklore.urban. Typical questions: why a keyboard is laid out the way it is, where a word like "bug" comes from, how you booted a machine from its front-panel switches. The answers frequently came from people who had sat at the console themselves, and a thread about a single instruction on a single processor could run for months.
For historians the archive became a primary source, because the eyewitnesses were ageing engineers who would otherwise never have written anything down. The group also kept the Jargon File and similar collections honest: a dubious anecdote got corrected quickly by someone who had actually been there. It is still going, greyer than ever.