Never in my life have I seen a more beautiful piece of software that scales well from very simple note taking to the most complex workflows you can imagine. Its community is awesome, you can meet the most nicest people.
To me, Org-mode solves things like simple todo lists, note taking, project management, spreadsheet calculations, calendar, contact management, file management, blogging system, knowledge base, quantified self, lifelogging, reference management, bookmark collection and so forth.
Org-mode has ended my search for most of the things I use a computer for. There’s nothing like it.