Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
Attendize Event Ticketing - Attendize is a free & open-source ticket selling and event management platform
GroupSpaces - Free Group Website, Membership Database, Listserv Mailing List, Online Group Management Software : GroupSpaces
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
The Groups Place - The Groups Place is a new alternative to Meetup.
pretix - pretix helps you to sell tickets for your event in an easy way. It supports multi-lingual events and provides a wide range of features