Calendarer!
Planning a party, a meeting, a get-together? You're in the right place. Create an event and invite your friends by entering their email addresses. When they visit the events page, they'll see a list of events which they've been invited to. If you'd like to create a Google Calendar event and invite your friends to it, allow this app to access your Google Calendar (by following the "Use Google Calendar" link), then click the "Add To Google Calendar" button.
This app is also an open source sample to show how your app engine app can add data to a Google service (like Google Calendar) using the Google Data APIs. For complete source code, look in the calendarer directory under google-app-engine-samples.
Create an Event
The inner workings of this app, and the general concepts involved in using the Google Data APIs in App Engine are explained in Using Google Data APIs on App Engine.