Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
SkillMint - Find, meet and rate people with common skills and interests. Community-driven rating system.
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
SurfersList - The ultimate online solution for finding, buying, and selling new and used surf equipment.
Google Groups - Google Groups allows you to create and participate in online forums and email-based groups with a rich experience for community conversations.
Eventzilla - Eventzilla lets you sell tickets online and manage attendees from one integrated application.