TeamSnap - Sports Team, Club & League Management Software & Apps

DownToDash - Meet fun people based on activities & discover events!

Spond - The easiest way to organize team sports or other type of group activities

Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.

Fullcourt - The social network for basketball players

Upcoming.org - A simple way to share and discover interesting events