Add a link to the current note to a daily note on a future date (or a past date, you time traveller).
Build advanced search queries.
Keep a workbench of knowledge materials.
Open or reveal the current note in DEVONthink.
Log your thoughts! Lumberjack adds URL commands to help you axe inefficiency and get right to writing.
Enable alt+↑ and alt+↓ keyboard navigation.
Pay attention to what you're paying attention to. Enlightenment adds a 'zen mode' for Preview, hiding the contents of your notes except for what's underneath your pointer.