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
DownToMeet - DownToMeet is a platform to organize, find, and attend group events.
Get Together - Get Together is an open source event manager for local communities.
Happer - Meet people & participate in activities, happier together