Google has announced the support of gadgets in Google Calendar. These gadgets “are special events that appear as icons above a day’s events.” When the user click on the icon, Google Calendar will include the content that the developer of the gadget specified in a small window.
You've seen them on iGoogle, in IBM WebSphere Portal, and now you can find them in your favorite online calendar ---"you can add this gadget that lets you check your horoscope from your calendar (don't worry, all the signs of the zodiac are supported). And if astrology isn't your thing, how about games? This Sudoku gadget was created by Alex Komoroske, one of our interns on the Docs & Spreadsheets team. If you add the gadget, you'll see that the daily puzzles get harder throughout the week."
Finally, if you like to keep up on celebrity birthdays, the birthday reminder will show you who you are idolizing that day:
Google, Google Calendar, Calendar Gadget, Calendar, Gadgets, iCalendar
You needn't a Googler to create a Calendar Gadget -- refer the new documentation to help you get your favorite Gadgets into Calendar. If you are want to write some code, then you can use GData or ICAL to add some fancy Calendar Gadget events to your calendar. You can try new "of-the-day" Calendar Gadget wizard, which makes it easy to take an existing Google Gadget and have it show up once a day, every day, just like the Horoscope and Sudoku Gadgets do.