Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
Wildcard - Discover, connect and meet people at communities
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
Shapr - A completely free way to build your professional network.
Bigtent - BigTent is a web-based organization and communication portal for groups.
LetsLunch - Do more with your lunch hour. Meet new people.