Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
Party with a Local - Party with a Local is a free app that connects travelers with locals that want to party.
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
Open Cult - A place for people with common interests to meet IRL.
Bigtent - BigTent is a web-based organization and communication portal for groups.
Cheers - Meet amazing people by sharing a toast with them