I love this little Export Event to Calendar widget on Meetup.com:
Most sites have a Bookmark / Share widget, because the viral return is obvious, but the viral nature of Add to Google Calendar is overlooked. I manage all of my invites and RSVPs via Google Calendar, so adding a button like this to your site makes it easier for me to invite my friends to your event. Every events site (ahem, ticketleap) should have this. Furthermore, every site that concerns my time should have this, eg, Kayak and Amtrak should use this widget for my departures and arrivals. Same for Bolt Bus. Landlords should use this for my rent reminders, grocery stores to remind me for milk even.