Email Marketing and subscription managers
MailChimp
Constant Contact
A Weber
Mailer Lite
Comparison of MailChimp and Mailer Lite - https://www.mailerlite.com/mailchimp-alternative
https://www.mailpoet.com/ (internal WP plug-in)
ConvertKit ($29/month) - Watched a StudioPress Webinar about this in May 2016
http://www.smartpassiveincome.com/why-i-switched-from-aweber-to-infusionsoft-to-convertkit/
https://wordpress.org/plugins/newsletter/