HEY - Email at its best, new from Basecamp.
SendRecurring - SendRecurring allows you to setup, send, and maintain scheduled email reminders.
Horde - Horde Groupware is a free, enterprise ready, browser based collaboration suite.
Recurrr - Send recurring emails
InstAddr - Disposable email address application.
FollowUpThen - The simplest way to schedule an email reminder