Discourse - Discourse is an open source discussion platform built for the next decade of the Internet.
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
GraphJS - Make any website social
Ticket Tribe - Sell tickets online with no fees per ticket
MyTalk - Build a community around your product
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