Joomla! London (https://www.joomlalondon.co.uk) talk about the basics of Email Marketing by Eoin Oliver of Square Balloon(https://www.squareballoon.co.uk), presented at February's #JUGL meetup.

Notes on Eoin’s talk on the Basics of Email Marketing
Presentation URL:
https://prezi.com/view/a74yKSdPscHbqoBIOBl2/
Countdown timers, Personalised Images, Photoshop Images, Live

Social Feeds, Rule Set Images, Data Source Images
https://niftyimages.com

Litmus made a nice blog showing gravatars
https://litmus.com/blog/behind-the-email-launching-community

Free Google Analytics Email Marketing Dashboard
https://www.campaignmonitor.com/blog/email-marketing/2014/11/free-google-analytics-dashboard-email-marketing/

Spam Words List (Google for more lists)
https://blog.hubspot.com/blog/tabid/6307/bid/30684/the-ultimate-list-of-email-spam-trigger-words.aspx

Subject Line Tester (use in Incognito mode if you can or else clear cookies)
http://www.subjectline.com/

Examples of Interactive Emails
http://freshinbox.com/blog/category/interactive-email-2/

Free HTML Email Boilerplate
https://github.com/uglyeoin/HTML-Email-Boilerplate
I will regularly update this please feedback or raise issues. It works with MailChimp.

Email Templates from other vendors
https://webdesign.tutsplus.com/tutorials/creating-a-future-proof-responsive-email-without-media-queries--cms-23919

mine is based on this:
https://foundation.zurb.com/emails.html Foundation is a web framework like Bootstrap, they made an email version

Open source email template builder without any HTML knowledge. https://mosaico.io/ export the HTML and import it to MailChimp etc.

Guide to what parts of CSS will and won’t work in HTML Emails
https://www.campaignmonitor.com/css/

email design inspiration
https://reallygoodemails.com/