Cardhop - The Mac contacts app you'll want to use.
Reminder - A better way to keep track of your reminders 📝
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.
DearEle - Email reminders made easy for everyone 💌
Contalist - The contact app for professionals and small businesses
FollowUpThen - The simplest way to schedule an email reminder