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.
Stuff - A platform for invitations to private and public events.
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
Guestboard - Lead powerfully-organized group events with Guestboard – a modular event platform that gives groups of 10-1000+ people a central space to coordinate, build excitement, and save money. No more crazy email threads. No more massive group texts.
Google Groups - Google Groups allows you to create and participate in online forums and email-based groups with a rich experience for community conversations.