Mobile-friendly, minimal, private, and open-source. Works great on your phone as an add-to-homescreen web app, and there is no lock-in with pre-paid hosting.
Reminduck - A simple reminder app made for elementaryOS with Vala and GTK. - matfantinel/reminduck
Todoist - Todoist is a to-do list that helps you get organized, at work and in life.
Reminder Commander - Reminder manager that reminds you about important events in advance.
Remember The Milk - Remember The Milk is a task and time management application for mobile devices.
Digital Clock 4 - Beautiful customizable clock with plugins
Google Tasks - to-do list app that integrates with Gmail and Google Calendar