Next Meeting is probably the single most useful experimental feature in Calendar, because it shows you exactly what event is coming up next, along with a clearly readable countdown timer so you don't miss it. It shows up as a widget in the sidebar, and the event displayed in the widget is highlighted the same colour as the calendar set that it came from.
Jump to Date
This feature allows you to quickly jump straight to any date in the past or future, without a flux capacitor. Not only is it great for jumping back to check what events happened on past dates, but enabling it along with the Year View feature (below) is a great way to get a handle on long-term planning.
This feature adds a simple, minimal world clock to the sidebar, and you get to pick and choose which cities to include in the list. It's especially useful for people trying to schedule events with co-workers who are located globally, and even better, it displays cities with dark backgrounds if it happens to be night time in their time zones.
Dim Future Repeating Events
This feature only applies to events that are slated for a specific time period, not a whole day. If there are recurring events scheduled for the same day as a one-off item, they'll dim slightly to make the slightly more important event stand out. Pretty useful if you've got a packed schedule and you're trying to skim through it for appointments.
Gentle Reminders was one of the first experiments to hit Calendar's Labs. When enabled, event reminders will flash in the browser tab and play a soft alarm sound, which is less obtrusive than a pop-up window. If you're using Chrome, a new option allows desktop notifications as well.
Automatically Declining Events
Calendar already offers an option to list yourself as busy during events, but that won't stop people from inviting you to anything and everything while you're gone. This feature handles those invites for you by simply declining each and every one if you tell it to do so.
Sharing calendars with friends or co-workers is great, but it only shares the events. This feature allows you to upload attachments to any event you want, and even grabs items from other Google services. The only catch is that if you're sharing something from Docs, you'll still have to actually set the doc itself to share, too.
There are a few more experimental features in Google Calendar's Labs settings, so be sure to check them out for yourself to see if any suit your workflow.