Really Good Emails - A large collection of good product email design.
Google Design Sprint Starter Kit - This Kit helps you to run your first Google Design Sprint.
Good Sales Emails - Best sales emails from great companies
Nawvel - Nawvel is a handy-dandy library of tried, tested & curated resources.
Great Email Copy - A dose of emails from around the web.
The Starter Kit - Curated resources for web developers and visual designers