DownToDash - Meet fun people based on activities & discover events!
Upcoming.org - A simple way to share and discover interesting events
Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
Party With Me - Find parties nearby
Party with a Local - Party with a Local is a free app that connects travelers with locals that want to party.
PlayWith - Connect around sports and games