This is the project I worked on at Google. I mainly told people I worked on Gmail because they're kinda related, but they are separate projects and I was on calendar.

I'm pretty excited about the product. Our hope was that it would really be the first calendar product ubiquitous enough to really get people using calendars in a big way, so wide-scale use was definitely one of our biggest goals. Some features that help with that:

• Event extraction. Google Calendar will scan your Gmail inbox for messages that contain some structured event information, extract that information, and place a link on the side of the message allowing you to add the extracted event to your calendar. This works with both free-form text and Outlook-style meeting invitations with an actual iCal attachment.
• Quick add. Near the top-left corner of Google Calendar is a "Quick add" link. You can use it to enter free-form text describing an event, and the corresponding event will be created for you.
• iCalendar and XAPI (CalRSS) feeds. Every calendar you create on Google Calendar can have public and private iCalendar and CalRSS feeds to make integration with 3rd-party applications (like Evolution, Apple iCal and Mozilla Calendar) easy. Google Calendar can also subuscribe to iCalendar feeds and import iCalendar files.
• Outside organisers. Using the iMIP standard, responding to meeting invitations send from Outlook, Hotmail, and Yahoo! Calendar works properly.

The features I worked on, for the morbidly curious:

• Calendar sharing and ACLs
• Multiple calendars and new calendar creation (this is definitely something you should use; you can create multiple calendars to have, say, a "work" calendar and a "personal" calendar)
• Some of the calendar feed stuff
• Some of the recurrence stuff
• General backend stuff

Anyway, go login and enjoy! If you have a Gmail account you can just login and start playing right away!