Simpletasks Dropbox vs Nirvanahq

Just like todo.txt cli, this is 100% free (although donations are accepted).

For those who don't care about syncing there is a cloudless version while people who run Owncloud may opt for the Owncloud version.

This not only tells one's calendar (and thus any devices that sync with your calendar) about the todo, but allows all synced calendars to play an audible alarm when the todo is scheduled to be finished.

Let's one sort to-dos by due date then priority

You can set how long a task will take and how much effort will it ask of you on a scale of 1 to 3.

A vital functionality in a GTD app.

Clean, intuitive user interaction. The design is very thoughtful. Somehow it makes Things feel clunky, despite obvious similarities of all GTD platforms.

The app can send you the most important items for the day in an email.

You can postpone a task. It helps not to clutter task lists up until you really need them.

Clean, intuitive interface — both desktop and mobile. Lots of intelligent design thinking behind this. Somehow makes other GTD apps like Things feel clunky, despite similarities.