Foundler - Online hackathon weekends. Attend every other week.
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
HackHub - Host tech events and hackathons with ease 👨💻🛠
Attendize Event Ticketing - Attendize is a free & open-source ticket selling and event management platform
Confs.tech - Open source list of tech conferences: JS / Ruby / Data etc.
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