Really Good Emails - A large collection of good product email design.
SwipeWell - The only tool dedicated to helping you save, organize, and reference marketing examples (so you never feel stumped).
Good Sales Emails - Best sales emails from great companies
AppGrowthLab (by SplitMetrics) - 55 app growth lessons from 30 app marketing experts
Great Email Copy - A dose of emails from around the web.
Marketing Examples - A gallery of real world marketing case studies. Updated every day.