TestSubject - Test how your email subject line displays on mobile devices.
Email Inspire - Discover the best of email design and marketing — Browse email newsletters from top brands.
Canned Emails - A minimal site with prewritten emails
EmailWay - Collection of the most insightful and informative email newsletters from across a wide spectrum of industries and interests
analyze.email - Test the spam-y-ness of your emails
Really Good Emails - A large collection of good product email design.