Google Calendar -- which was recently taken out of beta -- just introduced a brand new "Labs" feature. This strategy is a safe way for Google to test out beta features under a non-beta label.
Some of the new features you can currently find in Labs for Calendar include:
Brighten your day with a background image in your calendar. After you enable this feature, you'll find background image options on the General Settings page
Attach Google Docs
Attach a Google Docs word processing document, spreadsheet or presentation to your event. (Important: guests do not automatically have permission to view an attached document. You must share each attached document.
Keep track of the time around the world. Plus: when you click an event, you'll see the start time in each time zone as well.
Jump to date
Quickly navigate to dates in the distant future or past (actual time travel not yet supported).
Free or busy
See which of your friends are free or busy right now (requires that friends share their Google Calendars with you.
See what's coming up next in your calendar.
It's only a small list of available features right now -- but I'm looking forward to the day when they open up labs to third party developers in both Google Calendar and Gmail.