Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.

Cvent - Cvent's event management software provides event planners with a complete solution to increase event attendance and decrease event costs.

Eventbrite - Discover Great Events or Create Your Own & Sell Tickets

Eventzilla - Eventzilla lets you sell tickets online and manage attendees from one integrated application.

Bigtent - BigTent is a web-based organization and communication portal for groups.

Attendify - Develop relationships at the events that you attend by creating smaller events with just a few people.