B12 Bookings - Bookings scheduling software that integrates with your site
Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
Eventjoy - The Easiest Way To Organize An Event
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
Ploxel - Simple events management platform, sell tickets online.
Meetabit - All the local tech and startup meetups in one place - easy to organize & attend