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.

Ticket19 - Sell tickets for your events with minimal transaction fees.

Galette - Galette is a membership management web application towards non profit organizations.

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

Bigtent - BigTent is a web-based organization and communication portal for groups.