Mailtrack.io - The best email tracking solution I've found. Free
Horde - Horde Groupware is a free, enterprise ready, browser based collaboration suite.
ReadNotify - ReadNotify enables the user to know when email is sent and the reciever reads the email.
Roundcube - Web-based IMAP email client
Bananatag - Find out what happens to your emails after you press send! Add email tracking, attachment tracking, email scheduling and templates to your daily emails.
Mailo - Mailo is an email client where you can send and receive emails to and from anyone with an email address.