Mixmax Calendar - The fastest way to schedule meetings with anyone, anywhere
Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
Meetingbird for Gmail - View your calendar and schedule meetings right from Gmail
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
Calendly for Chrome - Schedule meetings without leaving your inbox
Bigtent - BigTent is a web-based organization and communication portal for groups.