25DaysOf - A customizable advent calendar, for everyone
Lightning Calendar - Organize your life — it’s about time!
Frozen Goodies - An advent calendar to get the chance to win daily prizes
Simple Calendar - A simple calendar with events (optional recurring and reminders), week numbers and a customizable...
Syndical - Automatically publish events across a range of platforms
GMinder - GMinder is an open source Windows System Tray app that pops up to remind you about Google Calendar events.