Mobile responsive email template
We worked on our main email template this last summer, using the 'Sample Responsive Design Newsletter - Single Column Template' from within Civi as a springboard.  We also created an account with MailChimp to see how MailChimp put their templates together.

As old as they are, HTML Tables are still the way to go because of how differently email clients handle display.  We used &lt;style&gt; tags as needed and used media queries within those &lt;style&gt; tags.  We ended up using a good bit of the CSS from our website to maintain our look; we just had to adapt our website's CSS to work with tables.

Here is an example mailing we did from last summer:
https://glenwoodcc.org/civicrm/mailing/view?reset=1&amp;id=77  We usually email short snippets and point back to our website, so we typically only have one main header image, one heading and then the content.  Our two column footer was a challenge to sort out responsively with tables, but we got there.  We also have a template that repeats the simple template so we could have multiple sections (image/heading/content) to the email.