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.
Todoist - Todoist is a to-do list that helps you get organized, at work and in life.
Bitrix24 - Bitrix24 is a free (for small businesses) social enterprise platform. It is a united work space which handles the many aspects of daily operations and tasks. Create your own social intranet in minutes!
Todo.txt - Track your tasks and projects in a plain text file, todo.txt. A todo.
Strikethru - Strikethru is a kind of to-do list notebook for managing work lists and staying updated with the important tasks.
Remember The Milk - Remember The Milk is a task and time management application for mobile devices.