Sortd - Rated the #1 App for Gmail
Horde - Horde Groupware is a free, enterprise ready, browser based collaboration suite.
KanbanMail - A Kanban board for your emails.
Mailo - Mailo is an email client where you can send and receive emails to and from anyone with an email address.
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.
Mailpile - Mailpile is a modern, fast web-mail client with user-friendly encryption and privacy features.