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.
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.
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.
Openbox - Openbox is a highly configurable, next generation window manager with extensive standards support.
Microsoft To-Do - Task management tool