ReadNotify - ReadNotify enables the user to know when email is sent and the reciever reads the email.
Roundcube - Web-based IMAP email client
GetNotify - GetNotify is a free email tracking software solution.
HEY - Email at its best, new from Basecamp.
Intelliverse Email Tracker - Intelliverse Email Tracker provides insights to increase the productivity of your emails.
Mailo - Mailo is an email client where you can send and receive emails to and from anyone with an email address.