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
Attendize Event Ticketing - Attendize is a free & open-source ticket selling and event management platform
Circle.so - Bring together your discussions, memberships, and content. Integrate a thriving community wherever your audience is, all under your own brand.
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