Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
Invitime - Meet new friends and challenge them to invite you
RSVP Keeper - Online RSVP's made easy
Bigtent - BigTent is a web-based organization and communication portal for groups.
Guestboard - Lead powerfully-organized group events with Guestboard – a modular event platform that gives groups of 10-1000+ people a central space to coordinate, build excitement, and save money. No more crazy email threads. No more massive group texts.