Greasemonkey - Customize the way a web page displays or behaves, by using small bits of JavaScript.
Recurrr - Send recurring emails
Violentmonkey - Violentmonkey is a userscript manager to support running userscripts in web pages.
Right Inbox - Schedule Emails in Gmail To Be Sent Later
GreaseKit - GreaseKit is a SIMBL plugin, that adds user scripting to Safari, Mailplane, Diet Pibb.
SendRecurring - SendRecurring allows you to setup, send, and maintain scheduled email reminders.