Todo.txt - Track your tasks and projects in a plain text file, todo.txt. A todo.
Lightning Calendar - Organize your life — it’s about time!
Remember The Milk - Remember The Milk is a task and time management application for mobile devices.
GMinder - GMinder is an open source Windows System Tray app that pops up to remind you about Google Calendar events.
Scheduled tasks - GNOME GUI for configuring a users' cron (automatic jobs). Some of its features are:
Calendar Lock PEA - Calendar Lock PEA is an encrypted desktop calendar with a day, week and month view. The shown calendars are never stored unencrypted on your disk, but exist only in the RAM. Platform-independent, Open Source, no installation or registration required.