This thread is used to record significant events in the life of Math.StackExchange. (There are also anniversary posts written from personal perspectives of contributing users.)
Where this idea comes from. Long ago, on a distant website, a user asked "What's the story behind MathOverflow?". Mariano Suárez-Alvarez replied: This is the canonical question that should be asked on the meta site! Anton Geraschenko wrote an informative answer, and other users added details. It seems that nobody asked such a canonical question here yet.
What should be recorded. Creation of the site (proposed, beta tested, graduated). Technological innovations like TeX support and chat. Chronology of moderators. Base-10 milestones ($10^k$ users, $10^k$ questions, etc). Probably something else that I can't think of now. But nothing subjective or potentially inflammatory.
How could it be useful? Newcomers to the site will have a way to satisfy natural curiosity. Wikipedia editors (and journalists, if they still exist) will have a convenient source of information about Math.SE (I'm sure there will be an article about Math.SE eventually; there is one about MathOverflow, and Math.SE reaches more people). Users reading old meta threads will be able to interpret them properly, knowing that a certain post was written when its author was/was not a moderator. Former moderators will have their work formally recognized.