Eventbrite is a software platform that event organizers use to sell tickets and registrations for their events…small and large.  They provides tools to organizers to create registration pages for their own events and promote them across multiple social networks.
There is no cost for the service provided the event is free to attend – there is a service fee when organizers charge for an event.  You can also use Eventbrite to discover new events in your area.   www.Eventbrite.com