Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
Pyro - Spotify music voting for parties
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
klaep - music collaboration - klaep is a social music collaboration network for musicians
Bigtent - BigTent is a web-based organization and communication portal for groups.
Madsounds - It’s like Discover Weekly, but daily.