Mobilizon - A free and federated tool to get our events off Facebook
Attendize Event Ticketing - Attendize is a free & open-source ticket selling and event management platform
Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
DownToMeet - DownToMeet is a platform to organize, find, and attend group events.
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