Fyne - The Fyne toolkit is an easy to learn, free and open source, platform for building graphical applications for desktop, mobile and beyond.
Really Good Emails - A large collection of good product email design.
Gio UI - Gio is an open source library for creating portable, immediate mode GUI programs for Android, iOS, Linux, Windows, macOS.
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.