Attendize Event Ticketing - Attendize is a free & open-source ticket selling and event management platform

Google Groups - Google Groups allows you to create and participate in online forums and email-based groups with a rich experience for community conversations.

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

Nabble - Nabble is a forum software that are embeddable and fully customizable with scripting language.

Ticket Tribe - Sell tickets online with no fees per ticket

Mighty Networks - Mighty Networks enables entrepreneurs, organizations, and companies to create and grow a community-powered brand.