Unlimited Email Tracker - Track opens and clicks, set reminders and delays in Gmail
Sortd - Rated the #1 App for Gmail
Mailtrack.io - The best email tracking solution I've found. Free
KanbanMail - A Kanban board for your emails.
Email Tracker - Find out if your emails were read
Trello - Infinitely flexible. Incredibly easy to use. Great mobile apps. It's free. Trello keeps track of everything, from the big picture to the minute details.