Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
Mobilizon - A free and federated tool to get our events off Facebook
Upcoming.org - A simple way to share and discover interesting events
Party with a Local - Party with a Local is a free app that connects travelers with locals that want to party.
Happer - Meet people & participate in activities, happier together
Party With Me - Find parties nearby