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.
Todo - Todo is a to-do and task list app.
Todoist - Todoist is a to-do list that helps you get organized, at work and in life.
Todorant - Tricks yourself into craving completing tasks
Remember The Milk - Remember The Milk is a task and time management application for mobile devices.
Be Focused by XWaveSoft - Simple Pomodoro timer in your Mac's menu bar
Google Tasks - to-do list app that integrates with Gmail and Google Calendar