Snailbox - Schedule your email at specific times and recover the time you're losing to your inbox.
InboxVudu - Spend 90% less time on e-mail.
Inbox When Ready - Protect your focus... hide your Gmail inbox by default
Handle - Your workflow companion seamlessly integrated w/ Google Apps
Clean Email - Clean Email is an online service that empowers you to take control of your mailbox.
Boomerang Respondable - Write perfect emails with Artificial Intelligence