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
Mobilizon - A free and federated tool to get our events off Facebook
The Groups Place - The Groups Place is a new alternative to Meetup.
DownToMeet - DownToMeet is a platform to organize, find, and attend group events.
Bylde - Bylde is a SaaS platform you can use to Start, Manage and Grow your groups and communities.