Org Mode

Possibly the killer app for Emacs

Org Mode Website

Org Mode Manual

Hyperlinking

desc	keystrokes
insert link	C-c C-l
learn to open a link	C-c C-o
jump back	C-c &

Interlinking

[[LINK][[[]]LINK][DESCRIPTION]]

Link abbreviations (custom link targets)

orgmode docs on link abbreviations

(setq org-link-abbrev-alist
      '(("duckduckgo" . "https://duckduckgo.com/?q=%s")
        ("wp"         . "https://en.wikipedia.org/wiki/%s")
        ("omap"       . "https://nominatim.openstreetmap.org/ui/search.html?q=%s&polygon=1")))

Agenda

desc	keystroke
open agenda mode	C-c a