Upcoming.org - A simple way to share and discover interesting events
Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
Mobilizon - A free and federated tool to get our events off Facebook
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
Openki - Platform where you can search, propose, take part, help organize or publish workshops, lectures...
Google Groups - Google Groups allows you to create and participate in online forums and email-based groups with a rich experience for community conversations.