Blogsend.io helps you retain your blog’s readers with the world’s simplest newsletter ever.
This plugin lets you painlessly implement it on your Wordpress blog. It creates a widget, which you can customise to your heart's desire.
See the directory page for this plugin for installation instructions: https://wordpress.org/plugins/blogsend-io/
The folder structure conforms to the way that Wordpress.org's plugin directory hosts files in SVN (Subversion).