Remember The Milk - Remember The Milk is a task and time management application for mobile devices.
Holiline Reminder - Holiline Reminder is both an event reminder and a calendar.
Notepad Checklist - A checklist made by notepad.pw, easy short and fun to use.
Trello - Infinitely flexible. Incredibly easy to use. Great mobile apps. It's free. Trello keeps track of everything, from the big picture to the minute details.
Todoist - Todoist is a to-do list that helps you get organized, at work and in life.
Reminduck - A simple reminder app made for elementaryOS with Vala and GTK. - matfantinel/reminduck