Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
Jersey Watch - Jersey Watch is sports management software specially designed for the teams and leagues for organizing players, managing payments, and effective communication.
GroupSpaces - Free Group Website, Membership Database, Listserv Mailing List, Online Group Management Software : GroupSpaces
TeamSnap - Sports Team, Club & League Management Software & Apps
The Groups Place - The Groups Place is a new alternative to Meetup.
SportNomad - Run new places. Discover yourself & world