Sortd - Rated the #1 App for Gmail
Templates by Email Monster - Hundreds of free, responsive email templates. Pick and edit.
KanbanMail - A Kanban board for your emails.
Really Good Emails - A large collection of good product email design.
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.
Good Email Copy - Email copy from great companies.