Sitecake - Drag and drop CMS for HTML websites. It's flat file CMS so it's pretty fast.
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
ClassicPress - The WordPress fork. No Gutenberg. Great future!
Cvent - Cvent's event management software provides event planners with a complete solution to increase event attendance and decrease event costs.
TYPO3 - TYPO3.com - Infos, SLAs, Extended Support Versions and more
Ticket Tailor - Ticket Tailor is an easy-to-use event ticketing platform that allows event organizers of all shapes and sizes to sell tickets online.