Does anyone recognize the plugin?
I wrote to the owner of the blog who said that it was no plugin and a default feature of wordpress but I have been playing with the latest wordpress and the twentyeleven theme (same as he is using) and I cannot see this functionality.
It is on this blog page LINK and it adds to the comment form
CHECKBOX Notify me of follow-up comments via email.
CHECKBOX Notify me of new posts via email
It’s called the Highlander Comments System and is available to wp.com users (i.e. people hosting their blogs on wordpress.com).
If you look at the URL of the site you provided, you can see that it is also hosted there: http://redwaspdesign.wordpress.com
As far as this plugin being available for non wp.com sites, it is discussed here: http://wordpress.org/support/topic/wordpresscom-highlander-comment-plugin
Basically, you can get it for a self-hosted site by installing Jetpack (http://jetpack.me/).
In their own words, “Jetpack supercharges your self‑hosted WordPress site with the awesome cloud power of WordPress.com”.
The plugin is then called “Jetpack Comments”