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.
GetReal - Meet new people, in person
Event Hunt - Startups and Tech Events Curated By Locals
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
Palaround - Meet people you should be friends with.