Web Applications
Emergent Mind (2014 and then resurrected in 2023) – EmergentMind.com
A/B Test Calculator (2015) – ABTestCalculator.com
Lean Domain Search (2012+) – LeanDomainSearch.com
Lean Designs (2011 – 2012)
Preceden (2009+) – Preceden.com
HNTrends (2009)
Domain Pigeon (2009)
Poker
ALL IN Expert (2008)
HUSNG Bot (2008)
Shortstacking Bot (2007)
Poker Blog (2006)
PokerShark (2006)
TetriNET
TetriNET Bot (2002)
AOL
AOL-Files (2000) – Archive – AOL Hackers Index
Modal Tool (2000)
Hacking iWin (2000)
Exploits Online (2000)
Meridian (1999)
Alpha32 (1998)
Revolution (1998)
Matt Mazur 
Perhaps this could be a useful tool in middle or high school in teaching students how to set up outlining for notes.
Thanks for this awesome list. Another great domain tool is Agile Domain Search (http://agiledomainsearch.com) – A free, powerful domain name generator that actually looks good. Simply enter a keyword to find great available domains for your website or startup.