Google Calendar - Spend less time managing your day & more time enjoying it
Todoist - Todoist is a to-do list that helps you get organized, at work and in life.
Simple Calendar - A simple calendar with events (optional recurring and reminders), week numbers and a customizable...
Joplin - Joplin is a free, open source note taking and to-do application, which can handle a large number of notes organised into notebooks. The notes are searchable, tagged and modified either from the applications directly or from your own text editor.
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.
Getting Things GNOME - Getting Things Gnome! is an organizer for the GNOME desktop environment.