WordPress Pretty Emails is an advanced and highly extend-able email template system for WordPress, There are 6 emails template by default on the plugin so you can select your desired template.

BuilderJS is a javascript plugin which provides a web user interface for building / editing HTML emails or web pages. BuilderJS is the easiest, quickest way to design elegant, mobile responsive emails or pages for your business. BuilderJS is made in pure javascript and HTML, making it easy to integrate with any web application no matter what the backend programming language is (Java, .Net, PHP, Ruby on Rails, Python, etc.)

 

Create a web app that outputs Google Sheet Data as JSON - Email a Google Doc as HTML code - Google Doc as HTML with Code

Delicious is a high-quality designed multipurpose e-newsletter PSD template with the coolest clean and simple style built for Companies, Enterprises, Businesses, Agencies, Magazines, Personal Blogs, E-commerce or Freelancers, who want to find a gorgeous, and clean solution to their e-mail marketing needs. You can choose what suits your company best and promote with a very beautiful looking template which is certain to attract the attention of your customers and inquire their curiosity. Delicious is truly pixel-perfect, build in 640px wide layout and considering the fresh standard newsletter guidelines, ready for coding to table-concept and responsive HTML for any kind of e-newsletter. You can change every elements of template quick and easy, Smart Objects for the fast image replace, you can scale, grow, distort, warp, transform the objects, ‘cause 100% PS ready vector shapes.