Mailtrack.io - The best email tracking solution I've found. Free

Intercom - Intercom is a customer relationship management and messaging tool for web businesses. Build relationships with users to create loyal customers.

ReadNotify - ReadNotify enables the user to know when email is sent and the reciever reads the email.

Zendesk - Zendesk is a beautiful, lightweight help-desk solution.

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.

Freshdesk - Freshdesk is a cloud-based customer support software that lets you support customers through traditional channels like phone and email, social channels like Facebook and Twitter, and your own branded community