Mail Boss Scheduler for Gmail

Mail Boss Scheduler is a Chrome extension companion to the MailBoss.org email scheduling service. It adds a "Schedule repeating" option next to the existing "Schedule send" option in Gmail's compose message window (click the down arrow on the "Send" button). The "Schedule repeating" option allows you to schedule recurring emails with just a few clicks. After installing this extension, you'll be taken to the Mail Boss web site in order to sign up for the service. It's free!

Screenshots from Chrome Web Store