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

Cloze - Smarter Relationship Management

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

Accompany - Accompany platform integrates with email, contacts, social feeds and more to keep you informed on your professional connections.

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.

SuiteFlow - SuiteFlow is a product to customer management solution offering a host of tools to manage inventory, contacts, invoices etc.