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
Bylde - Bylde is a SaaS platform you can use to Start, Manage and Grow your groups and communities.
GroupSpaces - Free Group Website, Membership Database, Listserv Mailing List, Online Group Management Software : GroupSpaces
Mobilizon - A free and federated tool to get our events off Facebook