DropTask VS GoAssign

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.

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

GTDNext - About Us.

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

Strikethru - Strikethru is a kind of to-do list notebook for managing work lists and staying updated with the important tasks.

Todoist - Todoist is a to-do list that helps you get organized, at work and in life.