Founders Nation - Find or become a startup cofounder
Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
CoFoundersLab - Meet 250,000+ entrepreneurs online
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
Findnlink - Find people to work with on your ideas.
Bigtent - BigTent is a web-based organization and communication portal for groups.