Really Good Emails - A large collection of good product email design.
The Creative Marketer - Techniques & tools for copywriters, marketers, and creatives
Good Sales Emails - Best sales emails from great companies
Zero to Marketing - Short case studies that help you get better at marketing
Great Email Copy - A dose of emails from around the web.
SwipeWell - The only tool dedicated to helping you save, organize, and reference marketing examples (so you never feel stumped).