Mobilizon - A free and federated tool to get our events off Facebook
Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
GroupSpaces - Free Group Website, Membership Database, Listserv Mailing List, Online Group Management Software : GroupSpaces
DownToMeet - DownToMeet is a platform to organize, find, and attend group events.
The Groups Place - The Groups Place is a new alternative to Meetup.
Facebook Local - Discover places to go and events happening near you