Cacher is the code snippet organizer for pro developers

MailChimp
Constant Contact
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/