ReadNotify - ReadNotify enables the user to know when email is sent and the reciever reads the email.
HEY - Email at its best, new from Basecamp.
GetNotify - GetNotify is a free email tracking software solution.
InstAddr - Disposable email address application.
Intelliverse Email Tracker - Intelliverse Email Tracker provides insights to increase the productivity of your emails.
Horde - Horde Groupware is a free, enterprise ready, browser based collaboration suite.