A comprehensive technical guide from Smashing Magazine on designing email newsletters for computer monitors and mobile devices. This article also includes several links to resources for a variety of technical experience levels:

Getting Started With HTML Email

• "Design and Build an Email Newsletter Without Losing Your Mind (or Soul)"
• "Rock Solid HTML Emails" An excellent starter tutorial.
• Premailer - For moving CSS inline.

HTML and CSS in Email Clients

• "Email Client Popularity: June 2011"
• "Guide to CSS Support in Email Clients"
• Email Standards Project

Mobile Email Design

• Style Campaign blog
• "What You Can Learn From Panic's Approach to Email Marketing"
• "Mobile Email Design in Practice: The New CampaignMonitor Newsletter"
• "Optimizing HTML Email for Mobile Using Progressive Disclosure"
• "Finger Snafu"
• "Make your HTML Email 5× More Mobile Friendly"

Email Design Inspiration and Templates

• CampaignMonitor's free templates
• ThemeForest's email template marketplace
• MailChimp's free templates
• CampaignMonitor's email design gallery
• Nice Emails' templates
• Creattica's email design gallery
• Beautiful Email Newsletters' design gallery