Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
RSVPify - A powerful and awesomely customizable platform for selling tickets online for your event.
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
Punchbowl - A party planning and greeting card website
Bigtent - BigTent is a web-based organization and communication portal for groups.
Whoozin.com - Free RSVP Online | Track RSVP’s and Sell Tickets for your events and recurring meetings.