Google Calendar implemented a basic snooze feature for pop-up notifications. You can click on "remind me again in 5 minutes" in Google Calendar's web interface.
"When alerted to an event via a pop-up notification and dismissing it, the calendar interface will show you an option to remind you again shortly. Click on that link if you want to be reminded of the event again," explains Google.
Another implementation is available in Google Talk Labs Edition, a Windows client for Google Talk that shows Google Calendar notifications.