Boomerang Respondable - Write perfect emails with Artificial Intelligence
Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
Mailbutler - Mailbutler adds lacking functionality to email clients and increases your productivity
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
Boomerang for Gmail - Boomerang for Gmail is a Firefox / Chrome plugin that lets you take control of when you send and receive email messages.
Bigtent - BigTent is a web-based organization and communication portal for groups.