Sortd - Rated the #1 App for Gmail
SendRecurring - SendRecurring allows you to setup, send, and maintain scheduled email reminders.
Drag for Gmail - Transform Gmail into organized To Do lists (like Trello)
Recurrr - Send recurring emails
Get My Receipts by cloudHQ - Organize your receipts in one click
Followup.cc - FollowUp for email is a lightweight productivity suite that keeps your conversations and tasks from slipping through the cracks. Set perfectly timed reminders that arrive at the top of your inbox, right when you need them.