"Today I'm gonna..."
One Big Thing is simplicity itself: a stripped back app whose main purpose is to get you focused on the one big task you'd like to achieve today. A simple but well executed idea, One Big Thing has you note down your one main goal, plus 3 other tasks you also hope to get done.
Each day, you get set notifications to remind to enter your tasks for the day - say, for when you're on the bus to work - and once you've done them, just tap 'Done' to complete them. Simples. For an IAP, you can add theme customization and some cute stickers too.
One Big Thing doesn't overload you with all of the many things on your to-do list. With only your most important tasks to deal with, scratching tasks off when they're done is all the more satisfying.
The layout is clean and uncluttered - a simple, but well executed idea.
Would be nice to be able to import tasks from apps like Trello, Slack etc. But that would probably taint the simplicity factor.