Google Calendar plugin can no longer be downloaded.
Not everyone is comfortable with the broad scope of Google, but if you don't mind one company holding a ton of information about you, a Google account is incredibly useful. If you use Google Calendar, which is a very well implemented calendar tool, you can now add a widget to Google desktop, and access it without your browser.
Google Calendar sits comfortably in the Google Desktop panel, as you would expect, and is pleasantly configurable. As well as setting up events, you can choose a daily, monthly or 'agenda' style view. It is completely synced with your Google account, so obviously you need to log in to make use of it.
While this is a cool widget to have, unfortunately it's not quite as functional as the browser version. Some of the options for setting up events are missing (you can't setup a repeat event). This is a surprising oversight, and perhaps it will be addressed in future versions.
Google Calendar is an attractive widget to have on your desktop, but it's a shame it's not fully featured.