Todoist - Todoist is a to-do list that helps you get organized, at work and in life.
Gnome To Do - GNOME To Do is a simplistic personal task manager designed to perfectly fit the GNOME desktop.
Remember The Milk - Remember The Milk is a task and time management application for mobile devices.
mCheck - A checklist application written in java with reminders and system tray support to improve...
Things - Things is an easy to use task manager.
DejaOffice - Super-charged calendar, contacts, tasks and notes apps for Android, iPhone and iPad.