i3 - A dynamic tiling window manager designed for X11, inspired by wmii, and written in C.
Todoist - Todoist is a to-do list that helps you get organized, at work and in life.
awesome - A dynamic window manager for the X Window System developed in the C and Lua programming languages.
Any.DO - The #1 task management app used by over 11 million people globally, Any.do is your free mobile and online task manager for Android, iPhone, Web and more.
dwm - dwm is a dynamic window manager for X. It manages windows in tiled, monocle and floating layouts. All of the layouts can be applied dynamically, optimising the environment for the application in use and the task performed.
Microsoft To-Do - Task management tool