Todo.txt - Track your tasks and projects in a plain text file, todo.txt. A todo.
Streaks - The to-do list that helps you form good habits.
Task Coach - Task Coach is a simple open source todo manager to keep track of personal tasks and todo lists.
Taskful - Deadlines, meet your match.
Strikethru - Strikethru is a kind of to-do list notebook for managing work lists and staying updated with the important tasks.
ToDoList - Windows desktop application to stay on top of your daily tasks.