Creative Market - Buy and sell handcrafted, mousemade design content like vector patterns, icons, photoshop brushes, fonts and more at Creative Market.
wxWidgets - wxWidgets: Cross-Platform GUI Library
SocialSizes - Sketch templates for sharing images & videos on social media
Electron - Build cross platform desktop apps with web technologies
Headers.me - Create social cover images that get you followers
GTK - GTK+ is a multi-platform toolkit for creating graphical user interfaces.