HEY - Email at its best, new from Basecamp.
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
InstAddr - Disposable email address application.
Clean Email - Clean Email is an online service that empowers you to take control of your mailbox.
Horde - Horde Groupware is a free, enterprise ready, browser based collaboration suite.
Nylas Mail - The Nylas Cloud API powers your application with email, calendar & contacts features. Built-in features for better email, calendar, and contact management.