Reminder - A better way to keep track of your reminders 📝
Sortd - Rated the #1 App for Gmail
DearEle - Email reminders made easy for everyone 💌
KanbanMail - A Kanban board for your emails.
FollowUpThen - The simplest way to schedule an email reminder
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.