org(-mode)
A lot is said online about org that confuses people unfamiliar with it. The best explanation I can give: Org is three things.
A markup language. A weird flavour of markdown, if you like.
Extremely good editor support inside emacs for that markup language. So good, that sometimes you are forgetting that it's just plain text.
A bunch of software bundled with emacs that can parse this markup language, filter and re-order it to display it in a useful manner.
I use org to keep myself organised (hence the name, I guess) and to plan my days.
I put notes on various topics, todo lists, appointments, tracking of certain achievements, documentation on various things and more into org. I can then instruct org to automatically dig up all the appointments and generate an agenda for the day / week. Or I can instruct it to dig up all todo items with a certain tag from a certain section.
Org is the reason people seem to believe I am well organized.
Some people keep multiple files with cross-links (like how I have set
up this digital garden!), however I prefer to have one single file,
~/Agenda/agenda.org. Inside I have a pretty boring hierarchy of topic
headers. However I do make use of tags more liberally, for example I
apply the :friends: tag to appointments that include my friends
regardless in which section I have put them.
Alternative implementations of org exist, but the one shipped with emacs is the only complete one. I am pondering how to procede with org if I couldn't use emacs anymore for some reason, because I prefer not to have a single point of failure for something this important. At least, since it's all plain text, I wouldn't loose the data!