Calendars & Tasks
Lightweight calendars and task managers.
The honest question is whether you'll maintain the system. OmniFocus and Amazing Marvin reward people who genuinely review their lists and punish everyone else, while Todoist, TickTick and Things stay usable after a week of neglect. Calendars are a separate purchase — Fantastical, Vimcal, Amie and Morgen are mostly about how fast you can find a slot and send it.
Editor's pick
Things is the one to try if your lists keep collapsing under their own structure. It deliberately holds less than OmniFocus, which is why people stay in it. Apple-only, and that's the trade.