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

EssentialPIM - EssentialPIM is a free Personal Information Manager that keeps up with the times and lets you manage appointments, tasks, notes, contacts, password entries and email messages across multiple devices and cloud applications.

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

Todo.txt - Track your tasks and projects in a plain text file, todo.txt. A todo.

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

Task Coach - Task Coach is a simple open source todo manager to keep track of personal tasks and todo lists.