Cardhop - The Mac contacts app you'll want to use.
Right Inbox - Schedule Emails in Gmail To Be Sent Later
Keep In touch - Keep in touch with thousands of people
Recurrr - Send recurring emails
Google Contacts - Google Contacts is Google's contact management tool that is available in its free email service Gmail, as a standalone service, and as a part of Google's business-oriented suite of web apps Google Apps.
FollowUpThen - The simplest way to schedule an email reminder