Todo.txt - Track your tasks and projects in a plain text file, todo.txt. A todo.
Reminder Commander - Reminder manager that reminds you about important events in advance.
Remember The Milk - Remember The Milk is a task and time management application for mobile devices.
Holiline Reminder - Holiline Reminder is both an event reminder and a calendar.
Scheduled tasks - GNOME GUI for configuring a users' cron (automatic jobs). Some of its features are:
DevTodo - To-do list for Chrome DevTools.