Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
Event Hunt - Startups and Tech Events Curated By Locals
Meetabit - All the local tech and startup meetups in one place - easy to organize & attend
Calendify - Calendify is an event community where you can share, publish, and subscribe to event calendars. It helps you to stay informed about what you are interested in and what is yet to come.
Mobilizon - A free and federated tool to get our events off Facebook