Punchbowl - A party planning and greeting card website
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
Whoozin.com - Free RSVP Online | Track RSVP’s and Sell Tickets for your events and recurring meetings.
Google Groups - Google Groups allows you to create and participate in online forums and email-based groups with a rich experience for community conversations.