Organizers

Agenda-dahenson is a free and open source very simple task manager to help you keep track of the tasks that matter most

CoreTime is a free and open source very simple clock/calendar application that supports alarm clock, reminders, has a stopwatch and timer

Reminduck is a free and open source simple reminder app

sleek is a free and open source, multi-platform task management program that uses todo.txt format

Super Productivity is a free and open source, cross-platform program for creating to-do lists and time tracking with integration of Jira, Github, and Gitlab

Journable – free and open source simple bullet journal (digital notebook)

Makagiga is a free and open source multi-platform simple software for creating to-do lists, notes, and reading RSS feeds

Purple Task is a free and open source simple software for managing tasks

OpenTodoList is a free, simple, cross-platform, open source software for creating notes and task lists