Org-mode made me switch to Emacs from Vim.

The one stupid productivity hack that has stuck with me for several years now is turning on a clock in org-mode for the task I'm working on, and then having to work focusedly on that task until I turn the clock off.

I do use org for keeping random diary notes and wiki-style notes and for doing quick spreadsheet stuff with the lightweight spreadsheet feature.

I found this massive article on workflows and customizations of org-mode pretty interesting.