Org mode
Used by 12 peopleorgmode.org ↗

“I take notes with Org mode but I feel like I'm only scratching the surface of its functionality.”
Zach Beane Interview, Developer (Common Lisp, Quicklisp) · via Uses This 
“Sitting on top of Emacs is Org mode.”
Katherine Cox-Buday Interview, Writer, computer scientist · via Uses This 
“I'm currently typing this in Emacs org-mode, which I use for nearly all kinds of documentation.”
Bob Mottram Interview, Software developer (Freedombone) · via Uses This 

“Org-mode as note tracker and TODO list manager (again, with mutt integration)”
Stefano Zacchiroli Interview, Computer science researcher, teacher, developer (Debian) · via Uses This 
“More recently I've experimented with org-mode, as it also exports documents nicely.”
Kieran Healy Interview, Sociologist · via Uses This