Planify
Planify is a task tracking program with Todoist and Nextcloud support for Linux. A free and open source program
AppFlowy
AppFlowy is a privacy-oriented workspace for notes, tasks, databases and more. Free and open source program
Osmo
Osmo is a handy personal organizer that includes calendar, task manager and address book modules. Free and open source program
Agenda-dahenson
Agenda-dahenson is a free and open source very simple task manager to help you keep track of the tasks that matter most
CoreTime
CoreTime is a free and open source very simple clock/calendar application that supports alarm clock, reminders, has a stopwatch and timer
Reminduck
Reminduck is a free and open source simple reminder app
Timecard
Timecard is a free and open source program that allows you to track how much time you spend on completing tasks
sleek
sleek is a free and open source, multi-platform task management program that uses todo.txt format
Super Productivity
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
Journable – free and open source simple bullet journal (digital notebook)