SalesHandy email tracking tool for Gmail allows you to know whether your sent emails have been read or not.

Boomerang adds scheduled sending and the easiest, most integrated email reminders to Gmail, helping you reach Inbox Zero.

Gmelius is a free browser extension available for Google Chrome, Mozilla Firefox and Opera that customizes the look and feel.

FollowUp for email is a lightweight productivity suite that keeps your conversations and tasks from slipping through the cracks.