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

Trello - Infinitely flexible. Incredibly easy to use. Great mobile apps. It's free. Trello keeps track of everything, from the big picture to the minute details.

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

Nifty - Manage projects, work, and communications in one place.

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

Asana - Asana project management is an effort to re-imagine how we work together, through modern productivity software. Fast and versatile, Asana helps individuals and groups get more done.