TickTick!

Todoist is a task manager thats useful, fast and easy to use.

Wekan, formerly LibreBoard, is an open-source and collaborative Trello -like kanban board application.

Take your tasks anywhere with this feature-packed application.

Take quick notes on Post-It(r) like sticky notes on Windows desktop, stick those notes to websites, documents & folders. Set reminder alarm to each sticky note. To avoid desktop clutter, you can organize those sticky notes inside folders. Send sticky notes to another computer over local network (LAN) or Internet, Sync sticky notes between computers using the Notezilla.Net cloud service, access your sticky notes on any smart-device such as iOS(iPhone/iPad), Android, Windows Phone, Blackberry etc. »

A simple but effective way to keep on top of your tasks, 10 languages.

An easy to use, web-based to-do list. Get organized, stay motivated, and be more productive.

Task Coach is a well-structured, portable, open source todo / task manager (supporting 50 languages). It grew out of frustration that well-known task managers, such as those provided with Outlook or Lotus Notes, do not provide facilities for composite tasks. Over time, Task Coach has added effort tracking, budgeting, notes, priority adjusting, reminders, and many other features.

Any.DO helps you get things done with your friends in a simple & elegant way. It's free, simple & fun.

Producteev is an online task management tool that serves as your full Productivity platform (Task and Project Management, Files Sharing, Internal Communication and Productivity Apps Aggregation).

Org-mode is for keeping notes, maintaining ToDo lists, doing project planning, and authoring with a fast and effective plain-text system.

Track your tasks and projects in a plain text file, todo.txt. A todo.txt is software and operating system agnostic; it's searchable, portable, lightweight and easily manipulated.

From a simple checklist to full-blown project management, 2Do is an incredibly powerful, award winning, task manager that lets you focus on what's important.