TicketHarbor - The safer Craigslist for P2P ticket exchanges
Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
Nightout - The standard in event ticketing and marketing by NIGHTOUT
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
Sporteventus for iOS - The worldwide search engine for sporting events and tickets
Bigtent - BigTent is a web-based organization and communication portal for groups.