Todo.txt!

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.

TickTick is your daily must-have to-do & task list to make all things done and get life well organized.

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.

EssentialPIM lets you manage appointments, tasks, notes, contacts, password entries and email messages across multiple devices and cloud applications.

Quire is a new-generation collaborative task management tool for organizing tasks in a tree-like structure wrapped in simple and user-friendly UI.