There's a lot of interesting forgotten stuff out there. These are my finds, some of which I'll eventually be putting into the Wikipedia:Historical archive.

Project pages

Some of the very oldest pages still exist in hidden form. They were what we'd now think of as policy or guidance pages, and were created before there were namespaces in what's now called article space. Later on editors cut-and-paste moved pages from their old CamelCase titles to new ones and converted the old titles into redirects. While articles usually had a straightforward transition in this way, the project page titles sometimes got re-redirected several times as project policies and guidelines evolved and lost any direct relation to their original content.

If you're looking at one now, it's almost certainly at least partially thanks to the tireless work of Graham87, who's spent years painstakingly reassembling early page histories from fragments scattered across a variety of inconsistent archive sources. Kudos.

• Wikipedia:What is a wiki (formerly at WhatIsaWiki) - January 2001
• SummarizeDiscussion - January 2001
• WikiPedians - January 2001
• BrilliantProse - January 2001 (see the current version for a detailed history of the precursor to Featured Articles)
• HowDoesOneEditaPage - February 2001 (shortly after its 18th birthday someone with no understanding of history tried to have it deleted. They failed)
• NamingConventions - February 2001
• WhatIsaWiki - February 2001
• DisRuption - March 2001

Pages containing or linking to obscure revisions

• User:Dbenbenn/bad user pages - features a lot of users who only edited in 2001
• Wikipedia talk:Unicode/Archive 1 - 2001-3
• User:Nemo bis/Bug 323 revisions - page revisions made inaccessible through the contributions page due to errors in the software transition of 2002
• Wikipedia:Usemod article histories - pages which were imported slightly later

Glitches

• Duplicates on AtlasShrugged, which was de-CamelCased in 2002:
  • 09:27, 26 February 2001‎ - old title / new title‎
  • 05:47, 1 February 2001 - old title / new title
• An edit from July 2002 attributed to "imported>10.58" instead of a user

Discussions

• meta:Meta:Historical/Archives index - lots of pages that were moved to Meta as a way of separating them from articles, before we had a project namespace for that exact purpose
• Wikipedia:Village pump/Wikipedia chat - 2002
• Wikipedia talk:MediaWiki (archive) - 2002
• User talk:Cumguzzler - January 2003
• Wikipedia:Articles for deletion/Wikicide - June 2003
• Wikipedia talk:Blocking policy/Bans and blocks - July 2003-March 2004
• User talk:EntmootsOfTrolls/on ban pages - August 2003
• User:Jimbo Wales/Ceasefire - August 2003
• User:Jimbo Wales/Pushing To 1.0/archive and Wikipedia talk:Pushing to 1.0/Archive 2 - 2004
• Wikipedia:Forum for Encyclopedic Standards - long-forgotten user group active in 2004-6

Tasks and reports

• Category talk:Newspapers/Tree - a manual attempt to build a tree of newspaper categories, December 2005
• User:R3m0t/Reports - an abandoned effort from 2005 to resolve orphaned talk pages
• Wikipedia:Changing attribution for an edit - from 2003-5 you could request that an edit you made while logged out be assigned to your account

Bugs and features

• Wikipedia:Squashed bugs (2002)
• Wikipedia:Phase II bug reports (2002-3)
• Wikipedia talk:Bugzilla (2003-4)
• Wikipedia:Ignored feature requests - 2002-4

Miscellany

• The state of the article "How-to" on 29 January 2003 (which survived in progressively smaller form until it was converted to a redirect in 2021) - a charming nutshell of the vibe of the project in its early days

See also

• Milowent's timeline of the very beginning of Wikipedia
• Graham87's history of vandalism reporting from 2001-4, at former process page Wikipedia:Requests for investigation
• Wikipedia:Wikipedia records