Senatus - Upvote for questions around a given topic.

Eventbrite - Discover Great Events or Create Your Own & Sell Tickets

Sli.do - Slido is the ultimate Q&A and polling platform for live and virtual meetings and events. It offers interactive Q&A, live polls and insights about your audience.

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

LiveQ - LiveQ is an interactive presentation software that lets audience can ask & vote questions anonymously. You can collect the best questions that you should answer. Loved by teachers, speakers, event owners.

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