Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
Discourse - Discourse is an open source discussion platform built for the next decade of the Internet.
Attendize Event Ticketing - Attendize is a free & open-source ticket selling and event management platform
GraphJS - Make any website social
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
MyTalk - Build a community around your product