Jira - The #1 software development tool used by agile teams. Jira Software is built for every member of your software team to plan, track, and release great software.
Todo.txt - Track your tasks and projects in a plain text file, todo.txt. A todo.
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.
Task Coach - Task Coach is a simple open source todo manager to keep track of personal tasks and todo lists.
Redmine - Flexible project management web application
Strikethru - Strikethru is a kind of to-do list notebook for managing work lists and staying updated with the important tasks.