KMailAssistant - Collaborative email management tool embedded into Outlook.

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

Freshdesk - Freshdesk is a cloud-based customer support software that lets you support customers through traditional channels like phone and email, social channels like Facebook and Twitter, and your own branded community

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

Hiver - Manage Customer Support, Sales and Projects right from your Gmail inbox. Collaborate with Shared Labels, Shared Contacts, email notes, email reminders and email templates

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