Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.

Google Groups - Google Groups allows you to create and participate in online forums and email-based groups with a rich experience for community conversations.

Calendify - Calendify is an event community where you can share, publish, and subscribe to event calendars. It helps you to stay informed about what you are interested in and what is yet to come.

Galette - Galette is a membership management web application towards non profit organizations.

Eventbrite - Discover Great Events or Create Your Own & Sell Tickets

Bigtent - BigTent is a web-based organization and communication portal for groups.