Boomerang for Gmail - Boomerang for Gmail is a Firefox / Chrome plugin that lets you take control of when you send and receive email messages.
MailChimp - MailChimp is the best way to design, send, and share email newsletters.
FollowUpThen - The simplest way to schedule an email reminder
SendInBlue - Innovative online Email Marketing solution to manage your contacts, create & send your newsletters and track your results. More than 80 000 clients. Best prices and attractive features.
Right Inbox - Schedule Emails in Gmail To Be Sent Later
Constant Contact - Constant Contact offers email marketing, social media marketing, online survey, event marketing, digital storefronts, and local deals tools.