Lightning Calendar - Organize your life — it’s about time!
Todoist - Todoist is a to-do list that helps you get organized, at work and in life.
Google Calendar - Spend less time managing your day & more time enjoying it
Gnome To Do - GNOME To Do is a simplistic personal task manager designed to perfectly fit the GNOME desktop.
Simple Calendar - A simple calendar with events (optional recurring and reminders), week numbers and a customizable...
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.