At PostageApp.com we focus on transactional emails, which these days are moving heavily towards HTML email as a standard now that deliverability is much more focused on IP reputation than content.
We provide a templating system which allows you to build and preview with separate HTML and CSS, and we compile to inline CSS at runtime to ensure compatibility with the majority of email clients.
The painful thing to remember is that html/css for email is a weird mix of HTML3 and HTML5 - meaning some major providers only allow for basic functionality, but other platforms support the bleeding edge while omitting some of the simpler aspects. In the end your demographics will strongly influence where you focus your efforts.
The templates from Zurb here are a great basis - as a start on a responsive email boilerplate this will be great value to the community - individuals using this for customization will hopefully push the development of these bases further.
I'm curious more along the lines of personalized emails to users of a service. As far as I know, MailChimp is more about blasting one message to lots of people. Unless I'm mistaken :)
Oh, okay. In that case you want to look at transactional email services. MailChimp offers that as well with https://mandrill.com, but there are a ton of other offerings as well like http://sendgrid.com.
