Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
Facebook Local - Discover places to go and events happening near you
Invitime - Meet new friends and challenge them to invite you
RSVP Keeper - Online RSVP's made easy
TechEvents.co - Tech and startup events aggregator