Tasks.org is described as 'Tasks is libre open-source software, based on the original source code from the popular Astrid Tasks & To Do List! Tasks is easy-to-use, feature-packed, flexible and customizable, and synchronizes with a variety of services' and is a very popular todo list manager in the office & productivity category. There are more than 100 alternatives to Tasks.org for a variety of platforms, including iPhone, Android, Web-based, iPad and Mac apps. The best Tasks.org alternative is Notion, which is free. Other great apps like Tasks.org are Super Productivity, Todoist, SPHW Pin and TickTick.
It's simple and takes less space. Subtasks are implemented by using tesk description.