Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
Upcoming.org - A simple way to share and discover interesting events
Mobilizon - A free and federated tool to get our events off Facebook
DownToMeet - DownToMeet is a platform to organize, find, and attend group events.
Agorakit - Agorakit is web-based, open source groupware for citizens.
Facebook Local - Discover places to go and events happening near you