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

Tinder - Tinder is how people meet. It's like real life, but better.

Eventbrite - Discover Great Events or Create Your Own & Sell Tickets

Pure - Spontaneous hangouts for singles

Bigtent - BigTent is a web-based organization and communication portal for groups.

Double - Tinder for double dates