Taskbook - Like Trello but for the Terminal

EssentialPIM - EssentialPIM is a free Personal Information Manager that keeps up with the times and lets you manage appointments, tasks, notes, contacts, password entries and email messages across multiple devices and cloud applications.

Taskwarrior - Taskwarrior is an ambitious project bringing sophisticated capabilities to a simple and elegant...

Todo.txt - Track your tasks and projects in a plain text file, todo.txt. A todo.

dstask - Command line todo list with git sync

Task Coach - Task Coach is a simple open source todo manager to keep track of personal tasks and todo lists.