Shortwave Bookmarklet - Shortwave is an extensible quick-search and shortcut bookmark.
Recurrr - Send recurring emails
Shortwave - Email smarter & faster with a reinvented experience for your Gmail
Right Inbox - Schedule Emails in Gmail To Be Sent Later
YubNub - YubNub is a networking web service with a command line–style interface.
SendRecurring - SendRecurring allows you to setup, send, and maintain scheduled email reminders.