Shapr - A completely free way to build your professional network.
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
Facebook Local - Discover places to go and events happening near you
Seatswap - Swap your seat on the fly.
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets