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

InsightSquared - #1 for Salesforce.com Pipeline forecasting, profitability analysis, activity tracking: all the small business intelligence you need. Works using CRM data and automatic syncing.

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

Aviso - Aviso provides predictive analytics software to help sales organizations optimize their performance and exceed revenue goals using machine learning algorithms and portfolio management techniques.

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.

Sidekick by Hubspot - Customer insight integrated with your email tools.