Hourglass - Hourglass is the most advanced simple countdown timer for Windows.
Right Inbox - Schedule Emails in Gmail To Be Sent Later
SnapTimer - SnapTimer is a simple, free, portable countdown timer for Windows.
Recurrr - Send recurring emails
wnr - Better than pomodoro, this timer app balances work and rest.
FollowUpThen - The simplest way to schedule an email reminder