TouchStream - Drip campaigns for Gmail
Recurrr - Send recurring emails
Signature.email - A powerful visual drag and drop email signature generator paired with the easiest way to maintain consistency for your whole team.
Right Inbox - Schedule Emails in Gmail To Be Sent Later
Elite Email - Drag & drop email builder for awesome newsletters
SendRecurring - SendRecurring allows you to setup, send, and maintain scheduled email reminders.