Your code could help people get to events on time, or ease coordinating their schedules. Google opened some of their Calendar interface to developers as a gadget Container, meaning developers can build features on top of Google's popular service.
The gadgets, announced on the Google Code blog, are shown in the right hand sidebar of the page. If they remind you of Facebook applications on your profile page, you're not alone. Similarly, users need to approve the gadgets and removing one is a quick click away.
The gadgets come with some helpful features. For example, OpenSocial is baked into the API, which gives you an open standards way to let your users interact with friends. Using OAuth is also simplified, so you'll also be able to incorporate third party applications.
The developer's guide has some calendar gadget examples. Because gadgets are written in XML, it's easy to learn by example.
If you're looking to access the data inside Calendar, you'll want the full API (our Google Calendar API profile). Getting at the data itself gives you more flexibility in how the data is displayed, but also means you're on the hook for the entire interface.
With the new gadget approach, you can add flair to a calendar with minimal coding. For inspiration we've got 31 Google Calendar mashups listed. What will you make?